Megaman and the official one both play brilliantly...
The X-Men mod feels a touch... samey; lots of enemies, but not much variation in them, but it runs ok - or at least, it did back in beta 0.2.
VOR GP32 demo is great - no music, but there are sound effects, and its possibly the best thing around for testing the new sprite engine. There's one point where I had probably 8 enemies on at once with no slowdown - which is darn good
I haven't been able to get the original (i.e. with music) Castlevania/VOR-PC demo/Nintendo vs Sega mods working yet, though I'm not sure why. TNMT likewise doesn't. Though bear in mind for all of those I've been using original paks, not modified, so modified ones (which have been stripped of music for size benefits) would probably play brilliantly. But I'm a perfectionist.
That said, I would love it if a future release had the ability to replace music with mods stored in the BORdata dir on a per-game basis (possibly outside of the .pak file)