I've migrated about ten sites in the last two months from 1.5 to 2.5 or 3.1. Sometimes i had problems with PHP-Settings or write permissions, but at least i always was able to migrate after fixing this things.
With the latest version 3.1.0 Database and Restful Method works without any problem. Done one migration last week and one today without any issue. So my vote is thumbs up!
So here a few details why i think so. The system is very easy to understand. Templates and Plugins have a very simple structure. I was able to write my own plugin in just one hour (it shows a thumbnail on a link).
You can do almost anything with Cobalt. I use it for a gallery, reviews, a list with icehockey stadiums (Googlemaps) and podcasts.
The only thing it lacks is a good import feature. So i can not import my Migthy Resources articles. But in there support forums they write that they are working on import feature.
Also, I have to say that the support is very responsive.