Hi Scooby,
Firstly I'm not sure if any game is WORTH £50, However both Faselei, and Evolution are very impressive for handheld games!
BUT they are both a bit weird, having their own peculiar control systems etc Evolution is a strange mix of RPG styles with an almost 'Final Fantasy' overall aproach (at least thats how it strikes me) as to if you will like it, it is a bit bizzare, very japanese, and a bit of an aquired taste. Much like it's dreamcast counterpart. On the upside, it's involving and different. And if you ancy somthing a bit deeper than 'Dark Arms' then get it.
Faselei on the other hand is perhaps the most over-rated game since Gran-Turismo! and it is certainly not worth the stupid amounts of money that it goes for on E-Bay. That said, it is a good game, the combat is turn based via programming chips that you aquire along the way, which can be combined into sequences of actions/attacks etc. This works quite well and eventually allows you to perform multiple tasks in one move. As has already been mentioned you can fully modify/customise your mech' to deal with the progressivley tougher challenges. All in all as with Evolution its a very impressive game for a handheld, and well worth having at a reasonable price!
Cotton on the other hand is WICKED. Forget what anyone says about it! If you want a sideways scrolling shoot-em-up on your NGPC, then this is the only one! Okay, it's not R-Type or Nemesis, In fact it's about as complicated as Scramble, But the graphics are excellent and its great fun to play!
As for fighters, Match of the Millenium without a doubt, all your Capcom fav's all your SNK fav's, fast as fuck, loads of sub games. its got it all, with bells on! it really is amazing. I didn't like 2D beat-em-ups untill I got MOTM, I've now bought all the rest on the NGPC (except KOF R2) and I must say they were all a dissapointment after MOTM. (although Rockman Battles is great fun)
As for puzzle games Puyo Pop is my fav' if you haven't played it think of 'kirby's ghost trap' on the SNES or 'Super Puzzle Fighter' on the playstation. However if you fancy somthing a bit more unusual try 'Ganbare Neo Poke Kun' which totally rules.
There are no crap games on the system, Take my advice and buy them all as and when you see them at reasonable prices!
cheers then!
Good luck choosing!