Clint I have Harrison Mixbus 3 as well and do use it. But I have found often I have to turn the tape saturation controls right down because I don't like what they do and what they do to the sound.
 
(Just out of interest I mastered some Hip Hop tracks recently and no matter what I did I could NOT get them sounding right in Harrison Mixbus. I ended up going back to Studio One 3 to master and it just sounded way better.  But that was very genre specific though. Other things I have mastered in Harrison just sounded way better as well)
 
But yes I do like it in just plain old summing mode. It seems to add a real nice sound over most things and it sounds better to me than most tape emulators.
 
I am having all sorts of issues with Mixbus 3. It is basically very unstable for me and I have stopped using it all together. Now I do have the 3.0 version and have not done any updates to it as yet but should I guess. They may have resolved a few things. I am starting to think that program is basically written for Linux and everything else is a compromise. Maybe I am wrong don't know.
 
 
 
 

I was having issues too. The interim release 1508 seems much better.
I think Harrison will find it's place with somethings here in the future. I like it. I may use it for mastering more than mixing though. We'll see. It was cheap.

I have used mixbus since ardour. Mixbus 3 is getting very close to becoming my main audio workstation. The sound quality is head and shoulders above most anything I have used to date. I am a rabid sonar user but if mixbus comes thru with a notation view, it will be a hard decision to make.
I do use mixbus 3 for quite a bit of mastering here as of late and NO, I don't use any tape saturation in 3 at all.
in sonar, I do use the console emulators quite a bit but not for saturation as much as tonal character for different styles of songs.

I do like what tape does to some specific tracks, so I think I'll experiment with trying it for effect at times. Going to try the Studer in place of the Slate though.
 
I have broken the habit of putting it on everything! :)

I use the tape emulation at the busses as well as at the master. The console emulations in every single track. Within each track the configuration varies a little bit to get a unique Sound from MY template.
BUT: I always do two mixes: One without and one with the effects.
Then I check the mixes at a different Location. The results vary: It is a nice touch if the material is something like Metal. If it is acoustic stuff the effects will "destroy" every win you had by using nice mics and preAmps.
My opinion: Great stuff if you want to go (a little bit) LoFi. And very helpful to design your own sound...