ok, the below is purely my own experience, others experience may differ
previously i was using MyHero 2.0.5(Hero Based Sense ROM) and it serves me quite well, everything seems to work normal, call, sms, LED, trackball lights, ringer, gps, wifi, bluetooth, data, camera and all.
Currently I'm using CursorSense 1.2.6.1(Magic Based Sense ROM) and its slightly smoother/snappier compare to MyHero.
only thing that is considered crippled is the Bluetooth File Transfer. As of now, there are no Sense ROM (cupcake 1.5) that support bluetooth file transfer.
about the lag, if you compare to iPhone then i would say yes, there is but its definitely NOT 30 seconds, not even close. To open up gallery is almost instance, but it may take 2-3 seconds to load all those thumbnails (currently i have about 50 over images and 7 videos on my sdcard) , to open a single image is almost instant

I've tried some 2.1 roms thats available, they feel even faster and snappier however certain feature still not fully available (LED, camera partially works etc etc). I'm waiting for a fully working 2.1 ROM then i'll migrate

Hope that helps