I know I have seen this mentioned by several, but never tried it until today. I have one project that I continually hem and haw on tempo. I work on it for a bit, brute force tempo, then put it aside for a few months, come back and want a different one.
 
I never really used the Loop Construction View at all, but it came to mind when dealing with the first reaction of shifting project tempo where all MIDI stays aligned and all audio goes all over. The solution was so simple I wish I had realized the usefulness of this long ago. Simply double clicking on each audio clip to view it in the Loop Construction view, selecting "Clip->Loop Properties" and checking "Stretch to Project Tempo" is all that is needed to get the same behavior from recorded audio as MIDI when playing with tempos.
 
Even nicer side effect of this is I can have just one project file now (I had several all saved at different bpm).

Good to know -  I don't use Loop Construction view either. This may be a handy little tool in the future, thanks for sharing!

When I tried it in the entry verison (X3) last year or so the results were really horrible (with many artifacts) because the entry version doesn't offer quality time-stretching (Audio Snap, etc.). I think I'll try it in Platinum and see if it's easier than Slip-Stretching. I just wonder where I can select the right argorithm (Radius Solo etc.) when using the Loop Construction view.

Even with Platinum, if changing tempos too aggressively this can fall apart the same way. For my application, I am only shifting by a few bpm. If I got too carried away with something I would end up re-tracking it most likely. I just tested a bit with more aggressive tempo changes and it seems that slip-stretching introduces fewer artifacts (I bounced the wav file before slip-stretching).
 
Another option is using an external wave editor, since it seems that Izotpope's Radius Mix is included inside many of them for this functionality. I doubled the tempo (87->174) of the same bounced file in Adobe Audition and it defaults to Radius Mix and seemed cleaner that slip-stretching.

Well, according to some tests I conducted after upgrading to Platinum, the quality of time-stretching differs significantly between Platinum (which offers both Audio Snap and Slip-Editing) and the entry version (which offers neither). In X3 base version, I changed a project tempo only moderately in the Loop Construction view (following the instructions in the ref. guide) but the results were really terrible and unusable. I know that render algorithms in Platinum (at least the default Radius Mix) also introduce some artifacts, but I hear much fewer artifacts. The Radius Solo algorithm performs really well. It can't be used for polyphonic data, though.

I rarely time stretch, but in the client session last night it all the sudden came in really handy.
 
I had prepared a draft song arrangement and tracked 3 rhythm guitars @152 BPM, recording the DI signal from the tube preamp and using Guitar Rig and Greg's new amp sims (which sound great BTW!) ... so the guy had redone the vocal line and now we needed a faster pace, hiking the tempo to 165 BPM ...
 
bass and drums come out of VSTs (in the arrangement phase at least), so no issue there to change tempo ... but for the guitars I had to find a real quick solution ... so I just ensured that guitar tracks start and end right on the beat (selected all and split off lead in and out), then changed the song tempo, and CTRL+dragged the end of the clips to the end of the measure while having snap set to 1/1 ...
 
since I did all that on the DI'ed signal all guitar FX is applied afterwards and it sounded fantastic (even using online methods) ... I did not even have to bounce anything for the entire session ...
 
so the whole thing took about a minute. the guy, who has been a live musician for 30 year, was blown away saying "you do everything in the box these days, don't you???"

That was exactly what I was doing before and it does work very well. The "issue" with this particular project is I am forcing myself to use Matrix View (self-imposed issue really), so the combination of being non-commital to tempo and all audio being in "8 bar chunks" was making tempo shifts a bit more painful for me.

The Loop Construction window doesn't use the iZotope algorithm. Although as Mettelus points out you can use it for small tempo shifts, for best fidelity you'd probably want to use that only as a draft, and use time-stretched slip editing once the tempo was finalized. 
 
The Loop Construction is primarily about editing audio for stretching, in particular, the ability to follow tempo and key changes. However as I've mentioned before, it's also handy when you want to do varispeed-type effects without artifacts (or at least, no worse artifacts than you would get by speeding up or slowing down tape).

If I am just working with scratch tracks during the writing phase I discovered that simply turning my audio clips into Groove Clips worked perfectly fine for not so drastic tempo changes. In the early phases of the project I have been working on (for far too long now) I had started at 180 BPM. It had a MIDI drum track, my scratch guitar tracks and I think an early bass track. It was dragging a little but I didn't want to retrack the outline based on the new tempo because I was sure that that was goign to be the final arrangement (it wasn't). IIRC I did a quick bounce/comp of the audio clips so I'd be working with full takes (instead of the composite clips) and simply did the right click > Groove Clip Looping (or whatever it is called) and made my tempo change from 180 to 190 and bingo bango... success! I made sure to render the tracks after I was happy that the new tempo worked and have been building the project around those prelim tracks ever since (although they are long archived in favor of final tracks).
 
However if I were to do this on FINAL tracks that have wild fluctuations (which I will have to do a lot of on an old album I need to fix) I would use other, more high quality methods such as the Timing Tool method, Audiosnap, the Loop Constructor or a combination of all three (and whatever other techniques I stumble across).
 
I however did not notice ANY noticeable artifacts or quality degradation simply using the Groove Clip/render method I described earlier. Very VERY powerful tools and plenty of ways to skin this particular cat. They just require a bit of brainwork.
 
I completely concur that these things all seem extremely intimidating until you actually do them. With a bit of studying behind you and some patience it's really quite easy to accomplish some CRAZY warping stuff. When I bought Sonar I didn't even know this type of thing was even possible.
 
Also, I posted that thread about Craig Anderton's X1 Advanced Workshop series. He goes into great detail about using the Loop Constructor to do all sorts of stuff. In particular, and probably the most useful of all the Looping and Warping chapter in the first vid is the one about getting into the Loop Constructor to ensure your clips a) have enough split points for quality stretching and b) how to create user defined splits for extremely PRECISE and ultra high quality stretching. Most of the time though just getting into the Loop Construction view and simply upping the split control to add more splits will work (like if you set the Splits setting from splitting at every 8th note to every 16th note the quality of the stretching will go up).
 
Cheers... and congrats Mettulus. I'm getting ready to do some tests with LC and the other warping possibilities to gain experience with it all. As I said I have an entire album that needs to be corrected and I want to be fluent in ALL the tools I have at my disposal. It is a very important project to me and I don't want to screw it up.

Oh... and I guess I should mention that you definitely want to snip the starts/ends of your clips at Measure points with Snap enabled if you are going to do the Groove Clip quick stretch method.
 
I think there is an option in the Clip Inspector under Groove Clips to set the Groove Clipping to cut the clip into time based splits (I'll check this out later) as well which might need to be looked at.