I drive an '07 AH Astra CDX...while you will be disappointed if you're coming from a Commodore, they really are, in my opinion at least, quite an underrated little car, and really quite a nice car to drive. Until I drove the Astra I was never really all that keen on getting a "buzzbox", though I have to say, it really has made a believer out of me. Sure, its no V8 HSV, but its no Hyundai Excel or Proton Satria (worst car I've ever driven) either.
As for things to watch out for, normally I'd suggest going with a 2L model rather than the 1.8, as the 1.8 is a bit of a slug for highway driving, but if its primarily for city driving I wouldn't worry too much. Another small issue is that if indeed you care, you'll need to make sure the Astra you get comes with the right head unit if you want to plug your iPod in - believe it or not the "premium" 6 disc in dash CD changer actually has no AUX input...I couldn't believe it either.