I would say the same. I've installed a couple of apps from both testing and devel, and so far I'm all good. But you should really be ready for serious issues. Make backups and don't install fishy stuff if you have an important call waiting . Also make backups, and be prepared to mess with the device if it would come to that.

Most important probably is to read up on the specific apps, on maemo.org/packages you can find any package and see the versions and changelogs. Usually there is also a forum thread, generally there will be posts if there is any serous risk, but take nothing for granted. As mentioned on the wiki testing and devel are for packages not ready for the end-user. I recomend that you should feel familiar with your phone and whit the filesystem, you know, just in case