I lived in San Diego (Mission Valley) from 2001 to 2003, and now I live in Irvine. Everything the previous posters said is true. There really are no bad areas in Irvine, but some are marginally more desirable than others. Like any area you move to, you should rent for a while and explore the different neighborhoods and see what they have to offer. Irvine is somewhat homogeneous, so don’t expect much difference between the neighborhoods. In fact, the only way to tell one from another is the signage. If you rent from the Irvine Company, they make it pretty easy to move among their various complexes, assuming you move up and not down. Living in Irvine is very convenient as it is a very large master planned community. Traffic in town flows well, but the freeways get congested just like everywhere else. The town itself is mostly flat, but you have mountain views all around. It is a great place to ride a bike if you are in to that.
I like Irvine, I am looking forward to prices coming down to a reasonable level. As SD Realtor pointed out, we are behind San Diego by about a year, so people are still in denial here that prices will decline at all. It is different here you know.