My lease in Crown Heights is just about up, so I can offer some help. I would advise not going past New York Avenue, and to stay north of Eastern Parkway. I lived north of the parkway for a year (St. Johns and Nostrand) & it's rapidly growing in those areas.
Currently, I have been living South of Eastern Parkway off Rogers Avenue, and while I've never had any problems, it's extremely quiet & not as "up and coming" as the north side. I don't really have anything bad to say, but I also don't have anything overwhelmingly good to say -- my apartment is cheap (a 1-bedroom), the building is beautiful, but the neighborhood is pretty run down. It was a sunny, beautiful day when I signed my lease so I'd advise going back to the location a few times before signing -- also, factor in where you spend most of your time. Cab rides from Manhattan are close to $30, and walking home at 2 am is really not the best idea.
Overall, I think it's getting better there, but I think it will be a few more years before it's great. I know a ton of people in the area, and they advise the same thing -- stay north of the parkway, and don't go past New York Ave. Hope this helps someone avoid leasing a year of unhappiness :)