There are a couple threads which talk about such things:
http://tinyurl.com/2yu2sr
http://tinyurl.com/2sjez4
My wife and I decided on the Feathered Friends Penguin (http://tinyurl.com/yokt65) and Toucan Deluxe ground sheet (http://featheredfriends.com/util/toucan.htm). We got ours with the new "Nanotech" fabric (not listed on the web page). Side Note: Their web page is ridiculously out of date and incorrect. Their products are much better than their web site (but I digress). We chose the heavier ground sheet for comfort reasons only. I plan on putting in a real review, but here is my quick summary:
General: 4 out of 5
Advantages:
1) Big puffy bag.
2) Both the bag and ground sheet weigh significantly less than the site says.
3) Quite roomy.
4) Design is solid and keeps you from slipping around or having any drafts via the edges.
Disadvantages:
1) There is a little spot at the bottom where you have to pull out a puffy section of the bag to prevent a draft.
2) Their is no draft tube to prevent gaps between the two people.
Room for Improvement: In general, the bag is meant to do double duty as both a shared bag and an extremely roomy singly person bag. I think this is a mistake. I'm not sure why anyone would want such a roomy bag (I certainly don't) unless car camping (in which case I'll bring along a really heavy (and cheap) synthetic bag). I think they could solve both the disadvantages mentioned by simply gearing this towards its primary use. They could also taper the bag a bit towards the toes and shave a little off the weight while still maintaining plenty of room.
Speaking of which, I believe Feathered Friends was in the process of making a tapered two person bag (which would be lighter and less roomy) but I forgot the link to it.