AutoCAD MEP - Good

Revit MEP - Bad

From what I've seen and everything that I've heard about Revit MEP it's way, WAAAAY too early to draw anything worthwhile. Just look at the piping. Three valves??! One of which is a suction valve (can't remember the other two) but if you're going to release a product, how 'bout throwing in a more common valve like a ball or butterfly valve.
One BIG PITA that I noticed with R-MEP is supply diffusers. Insert one.... what's it's elevation (or 'offset' as they call it)? ZEE-FREAKIN'-RO! You don't have an option of setting an elevation when you insert it so you'll either have to 1) change them as you go or 2) if you typically place them at, say 8-feet, you can modify the diffuser part or family (whatever the hell they call it) and move it in the Z plane 8' (or whatever you'd like).
I don't think R-MEP 2008 has flat-oval ductwork either, does it?
The electrical seems to be pretty good (from what I've seen) but it's not nearly complete as it should be. Theres still a LOT of content missing. They've got what... ONE type of receptacle and one type of switch?? Do they expect you to manually label the various types that you could have on a job?
We don't plan on moving to R-MEP any time soon simply because of the lack of content available. AutoCAD MEP is more developed and has more content. Oh, and I wouldn't expect Trane or Carrier to start developing any content for R-MEP any time soon.... they barely have anything for A-MEP.
R-MEP also makes it somewhat more difficult to create your own custom content.
As for Navisworks (or Jetstream) that would most likely be the way to go to determine any collisions. You can export your R-MEP or A-MEP drawing as 3D objects and essentially import them into Navisworks, run a check and get detailed reports for each collision. It also has a really nice walk-thru feature. I'm hoping we buy a seat or two when Navisworks is released by Autodesk (fingers crossed).