I have never transferred a pokemon into another gen. before gen 5 I only owned gen 1 games.
Does transferring take an eternity? if yes, then I'll only trade over my main team, my legendaries, the special ones (starters, eevees, etc) and a few others.
yes it does as you can only transfer 6 at a time and have to play a min game each time. Diamond and plat only let you transfer 6 pokemon a day from each of your gen 3 carts. Transfering to Black however doesn't have a limit, but requires that you have 2 DS.
Pokemon that know HMs can't be transferred so you also have to take the time to go to the move deleater and delete the HM moves.
A think a bunch of pokemon in gen 4 and 5 are only available via import. Also some pokemon are easier to acquire in gen 3 (i.e. Milotic).
"What is better. To be born good or to overcome your evil nature through great effort." - Paarthurnax Gt: Ghosta2