Chose a Nokia for my next contract phone and it came with £20 credit to spend on the app store. Though I didn't bother with the app store on my last store so now sure what sort of stuff is actually worth buying. The choice of games/apps seems fairly limited, just not sure which of the few are actually worthwhile.
I've purchased quite a few of the Xbox enabled games. Not sure what sort of stuff you like, but I can happily recommend the following.
Halo: Spartan Assault Skulls Of The Shogun Civilization Revolution Plants vs Zombies Carcassone Feed Me Oil Pac-Man CE DX Rayman Jungle Run Contre Jour Zombies (on teh phone) Max & The Magic Marker GeoDefense GeoDefense Swarm
All the Xbox titles have trials, so give them a go. Which Lumia did you go for?
EDIT - Always worth checking out the deal of the week too. This week is Mass Effect: Infiltrator, down from £4.49 to £1.99.