I summarized my experience with importing characters in this post: http://www.gog.com/en/forum/might_and_magic_series/mm2_import_characters_or_start_over/post11
Basically, you can get all your characters to start MM2 with 20 in every single stat if you import after you've raised your stats in MM1. So that's an advantage to waiting. But really it depends on what you want to do. Do you want to finish MM1, or do you want to just skip to MM2? If you want to finish MM1, I would do so and then import, but if not just go ahead and import once they hit level 6. Personally I enjoyed MM1 a lot so I'd recommend sticking it out, but by all means do whatever you want to do.
As kmonster said, you'll get to MUCH higher levels in MM2 so importing will really only affect the start of the game. It's nice to skip over the early level grinding though.
One final thing to consider: there are two new character classes in MM2, the barbarian and the ninja. To use one of these you have to create a new character; however you can also hire extra party members of these classes if you wish. I ended up using my full MM1 party because I'd grown attached to them, but if the new classes sound cool you might consider making some new characters also.