Oh how I hate Take Lanes. 
 
I know, I know... "get used to the new workflow and choirs will sing from the heavens" ... but come on. Didn't any Sonar developer take the Take Lanes implementation, give it to someone who liked layers and say "Try this, what do you think?"
 
I really do want to learn this mysterious and confusing feature. And I will. But to start right off the bat without getting this post too long, I have two simple questions:
 
1. How do I consolidate all the takes into a humane number of lanes, so I'm not just seeing empty row after empty row?
2. I'm not a dumb guy. Why is something so frickin' obvious as question 1 so frickin' impossible to figure out? I went into the online help and searched on "collapse take lanes" and "consolidate take lanes" and scrolled all over and couldn't find it. Didn't anyone think "Huh, there is a lot of empty space here. Let's make it obvious how to get rid of it"?
 
So before someone smugly types the answer, take a moment with X3 and pretend you don't know. Click on things. Right-click on things. I defy you to find it.

There is a 'Remove Empty Take Lanes option in the Track view of the track you are working on. Right Click on the track in the clips pane, NOT TAKE LANE, and you will get that option. That should help a little bit.
 
I feel your pain...

"Rebuild layers" or a comparable function was removed and there have been some discouraging words from cakewalk about returning it, although it seems fairly simple to implement so maybe it'll return.
 
 
 
I think many of the issues that have come up with Lanes (though not this one) are actually with the comping tool rather than Lanes themselves.  One change I would love to see is a Smart Tool without the comping features - frankly I think that should be the default Smart tool, and that a special Comping Smart tool should be selectable from the Smart tool dropdown list, but one way or another it would take a lot of the mystery out of the process if the comping behavior was optional with the smart tool.

1. Right click on the clips in the parent track (the main track where the comp is built up) and select Delete Empty Layers
 
2. I don't think there is a way to make all the non overlapping clips instantly occupy a single lane and remain separated like we used to be able to with Layers BUT it doesn't matter. Again right click the parent track and select Flatten Comp and all the currently active clips will create one continuous clip in a new lane and mute everything else. I THINK at this point you can remove all the scattered clips that created the new comp can be removed by again right clicking the parent track and selecting Delete Muted Clips (but I am not certain).
 
Yes that last bit is not exactly the same as the Consolidate Layers feature but that was buggy as heck anyway and there are other ways to do this type of thing VIA Bounce to Clip(s) and still get the same result but without all the unnecessary gaps.
 
Hopefully that isn't too smug for you.

1. - in X3 the name of that process is what I posted earlier. It used to be delete empty layers. There are no such things as layers any more. RIP layers...

Aha! See, Flatten Comp (flatten? that's the most intuitive verb they could come up with?) is NOT AVAILABLE if, like me, your track is displaying PRV instead of clips. If, like me, a track is in PRV mode, and you open the Take Lanes feature, there is no way I can tell to "flatten" them. 
 
So why not? Never mind, at least now I know how to "flatten" them. Thanks all!
 
Also, why isn't it called "Flatten Take Lanes"? The button to open them is called Take Lanes. Why when you want to consolidate them, are you expected to know it is now called "Comp"? Wow, terrible UX right there.

Well there is an expectation that with an entirely new workflow/feature set you'd spend a little time learning the new terms and what they do. The other methods are still there (except for Layers) so if you don't want to spend the time learning the new stuff just change the mode to the more traditional method. That's what I did while arranging my current project. I set it to Sound On Sound and worked how I normally would.

Oh and PRV editing isn't really related to Comping. You use comping in the Clips Pane. Then after you Flatten the comp you open the resulting clip in the PRV and provided you have Hide Muted Clips selected in the PRV Views menu only the notes in the resulting comp will be visible. THEN you do your MIDI edits in the PRV. In fact you don't even have to use Flatten to do this because during the comping phase everything except the promoted clips are automatically muted. Still it's less confusing to work with a single clip.
 
So basically do your comping in the Clips Pane.
 
Do your MIDI edits in the PRV.
 
This method of comping does NOT occur in the PRV.
 
I actually used build up my drum tracks in the PRV by lassoing, copying and dragging sections around but that is WAY more work and far more annoying than just putting things together in the Clips pane.
 

Flatten Comp does a mixdown though, doesn't it?
 
If you're working with clips which are not a comp, or even if you have comped something but would rather retain the original clips but simply have them take up less space, flatten comp won't help.
 
Rebuild Layers worked pretty well to help use space efficiently when working with clips.  The only way in which it was buggy was that it wouldn't always keep the layers in the same order, and it was a mystery how it behaved.
 
Actually one thing that is greatly improved, and seems related to the rebuild-layers-ordering problem, is the way the track now shows the actual audio being heard rather than a hodge-podge of muted and unmuted clips.  It could use a bit more polish when displaying overlapping clips, but it's enough to make working with clips better in X3 than pre-Lanes Sonar, in my view.
 
I know it seems hard to understand how different people use the same tools differently, but for people who are working with clips outside of the standard comping workflow, a return of the "rebuild" functionality would be really helpful.

Right but Flatten creates a new lane for the flattened comp to live in while retaining the original clips and because the comping tool splits, crops and x-fades any benefits that I can think of where you would require the original splits to appear in one lane/layer are negated. All you have to do is swipe the clip and the sections are there. Drag up/down for X-Fades. Drag edges to crop the splits (on the correct side of course) to create holes if desired.
 
I'm just not seeing the need for having a bunch of scattered clips getting crammed into one lane anymore. Before, yes, because it was more cumbersome to split things back up.
 
You're right though that perhaps I'm not seeing something you are however even though it is radically different from what I can tell everything that could be achieved before by the Consolidate Layers option is covered and then some. It is obsolete at this point.