I've got a MSAR E4 (takes AR15 mags).
I had a real love/hate relationship with bullpups...but I bought one to see what they were like. Here's my podcast on my saga of the search for a bullpup and what I like/don't like about them.
http://gunreviewpodcast.com/category/long-gun/rifles/
Frankly, the best place I've found to learn about them (and other bullpups) is over on the AR15.com forum, Bullpups section
http://www.ar15.com/forums/forum.html?b=6&f=43
MSAR has about 10,000 rifles under their belt. Their early ones had some teething issues...not uncommon for any new product. They seem to have the bugs worked out and have built and excellent reputation for quality, customer service and new product introductions. Given that the company is the firearms division of Microtech blades, they aren't "fly by night" and have some longevity.
Positives
+ Compact
+ Handy
+ Less muzzle rise (because of the geometry -- smaller cantilever)
+ Longer barrel for same overall length
Negative
+ Triggers not as good (long linkage)
+ Slow mag changes
+ Harder to make ambidexterous (except with the forward / bottom ejectors like the FN2000 or P90)
In general, these rifles do have some advantages, especially for CQB. They will not, however, make me give up conventional configurations.