Rails? MBP or better, for me. Running unit tests on the Air kicks my fan on and takes many seconds just to start up the harness. You need to think of available RAM also, if you're using a fat IDE like Eclipse or RubyMine.
A way around this is to simply SSH into a bigger box to work on. Then it's GREAT. If you use Fuse w/ SSHFS, it's powerful enough for a fat IDE... but I still use TextMate or Vim over SSH and run my tests on command line remotely on the Air. It saves days of productivity.
Which year Air do you have? When I fire up a Rails test suite on mine (hundreds of tests, many with JS through poltergeist) it doesn't stutter or enable its fan.
A way around this is to simply SSH into a bigger box to work on. Then it's GREAT. If you use Fuse w/ SSHFS, it's powerful enough for a fat IDE... but I still use TextMate or Vim over SSH and run my tests on command line remotely on the Air. It saves days of productivity.