I see that 2.5 is stable now. In general I do not to use the latest verstion of limesurvey, but the version before that. Does anyone know wheter 2.5 is really stable, or is it (for serious use) better to wait some time?
Frans (DigiPsy)

Frans,
if you read this post (from LS core dev team) you will understand that you can install a fresh new 2.5 (stable ?) platform but not update from a previous version to 2.5 . The difference of the approach related to "release" and "update" is clear: are you ready to loose something?

To be completely honest, I personally would not use 2.5 for production just yet. If you look around in the forum you will see that there are still some minor or major hickups. They might or might not impact you. I'd say to wait a few weeks.

What I usually do is to run old and new in parallel. It doesn't really cost you additional and it shouldn't take too many resources anyway. This way you can try LS 2.5 and see if it meats your needs.

What I usually do is leave the old surveys on the old version, but once I feel the new version is good to go and has everything I need I start building new surveys only on the new version. If something goes wrong I could still export the survey and run it on the old server.

I think unfortunately 2.5, under the premise that it is actually just 2.06 with the interface changed, was not tested thoroughly enough and my feeling is that unfortunately there was not enough feedback on the beta versions and not all of the small bugs have been found. That said, this is my impression based on the number of posts in the forum. I didn't have enough time to test everything myself yet.

I think the new interface is great (sure, there things to improve, but isn't there always). Once the little hickups are solved, this is a great step forward for Limesurvey, especially with the new responsive templates.