I've got a DJM 600, just so easy to use, the knobs and faders are all a good size and really well spaced out. These things are built like tanks too! I've used Allen & Heath in clubs before and I find them a bit fussy to be honest (wasn't that impressed with the Penny and Giles crossfader) but the sound quality is better than the DJM range. Anyway A&H are bloody expensive too!
You won't be disappointed with a DJM mixer unless you're a big time scratch DJ, in which case you I reckon you should be looking at Vestax, Ecler or Rane.
Also nothing wrong with buying a second hand mixer, just make sure it all works before handing over the cash.