I'm working on a 2 to 3 migration right now. It's not too bad. I have a decent framework of tests and have been using mostly 3 compatible code in 2.7...
That said I asked myself a lot of times during the process, "Why am I doing this again?" Every change is a chance to introduce a bug and I can't think of a single benefit to this code base to migrating. IT might say, "2.7 isn't being updated and is therefore a security vulnerability." But this is a desktop application running behind a corporate firewall.... the risk incurred is so minuscule, if we actually did a cost/benefit analysis I don't see how we could justify the handful of staff days required to make the migration.