Better discharge rate, more forgiving, not as fussy to balance all the time, you can forget about them for years and then just cycle them a couple of times and they come back to life, not so sensitive to heat.
Only one downside (important one though). They don't come in high capacity to match Nimh. Actually I have never seen anything above 2400mAh and that is barely enough for a 5 minute race in stock (27 turn) on a twisty track (a lot of brake/acceleration which sucks a lot more energy than an open high speed track). For silvercan and possibly mini I would just race my old Nicd (the more so since their weight would help with traction issues as well).
Team Greasy Weasel
The best upgrade to any car is some driver skill.