Ok, I know, migrating to kmail2 is old news now. But only today I decided to try migrating to kmail2. Gentoo is going to remove kmail1 from their repository in a few months so I did not have much of a choice.
My kmail1 configuration included one pop3 account, one gmail account and some local folders. The migration process finished and then nepomuk started indexing my e-mails (my laptop's fan screamed for several hours hehe). During the indexing process kmail2 exited with the message "Failed to fetch the resource collection". After some search on google I found this blog that explained how to fix that (it is simple, really). Then I had a problem with e-mails not being sent. The error message talked about mailfilteragent, which is installed by kmail2 itself but is started by akonadi. Restarting akonadi made it recognise the new agent and the problem was fixed.
After importing my local folders manually (the migration process did not do that) I configured my mail filters and waited until nepomuk finished indexing my e-mails.
So far so good using kmail2, no crashes and everything works almost like the old kmail1. One exception was the "go to the next unread message when opening a folder" option that is disabled by default. Alex Fistas helped me with that and now I can do with kmail2 everything I used to do with kmail1.
Update: well, the "so far so good" did not last a day :-/ I have found this really annoying bug and submitted a patch to reviewborad to fix it. With luck this will enter 4.10.3.