I'm not all that experienced with it, I traded in an Xbox360 Elite 120GB about 18(ish) months ago with 2 controllers and I don't know how many games, maybe 15, to Game in the UK, and I got about £200 out of it. I traded it in against a new PS3 slim 320GB boxed with Black Ops and I grabbed a few used PS3 titles with the excess credit (they had a promotion on trading 360's against PS3's so I probably got more than most for my shit).
Most of the games I traded in were pretty old, but I actually got more back from Halo Reach than I paid, most of the rates were pretty shitty, but most understandable for the age/popularity of the titles.
I also boxed up a shit load of CD's, dvd's and PS2 games to Music Magpie when I moved last year purely as a quick way of getting rid of stuff quickly, the return on that is pretty bad, but it is essentially for shifting a load of shit quickly. Had I ebay'd everything I'd probably have gotten 2 or 3 times the value, but I sent in about 60 items, that would have taken days/weeks to get on Ebay and some of it might not have shifted.
I never traded in before that. I used to really like collecting boxes. That all changed in 2007 whn I mostly became a PC guy.
I'm usually ok with buying used, sometimes it's the best way to get older console titles (sometimes the ONLY way). A couple of times I've been given used discs in horrendous condition. Most recently I got a copy of Uncharted 1 from Game's website that was scratched up to fuck and wouldn't run (blu-ray actually have great scratch resistant coating so someone was really shit with their discs), another time I bought a game from Gamestation that was covered in some weird shit, it came off but if I'd noticed before I got it home I'd have told them to get fucked. Basically ask to see the disc before you hand over the cash.
Personally I don't see trade-ins as particularly "worth it" unless you really want to get rid of things. The only trade-in deals that seem to make any sense are promotions where you get a new release for like £5 if you trade in certain other games - although these tend to be extremely new and you'd have to be the sort of person who burns through games exceptionally quickly for those schemes to be of any value.