We still have a forum on XF 1.5 that I'll finally be upgrading to 2.1 in the next few weeks. (Long story, involving add-ons we needed but couldn't get right away.) Let's just say that in this final phase, I have been tweaking and testing the 2.1 upgrade for several months (on a private testing instance of the forum) and working out every bug that I can before we go live. And I'm anxious to get upgraded to 2.2 and start working on it, but I have to wait until we launch 2.1. On my smaller forums it is not a big deal, but I am working on one of ours for 2.1 and assisting with another, and they are both high-traffic forums. There is no way I'd blindly start upgrading anything without a lot of testing beforehand! Install it, choose the third party theme you want to use, install your add-ons, make adjustments and theme variations and test them relentlessly. That's the only way forward for me.