Personally, I don't much like the Firestorm viewer. I find the current version leaches memory after a while and can get sluggish. It is rather overloaded for my needs anyway and I don't have any real use for MOAP that a browser can't serve better opening to a full screen and left open to tabbed pages in the background. But, each to their own needs and preferences on that one. And, of course, Firestorm is more geared to serving Second Life in it's current form more than Opensim so I would rather wait and see what the Firestorm team do with a forked OS version. Teapot and Imprudence still serve most of my needs currently and I would rather see Teapot developed further with XML Import/Export added. I like the way it's internal browser handle's web pages too (they all seem to do it is slightly different ) and the HOP URL protocol works well for grid addresses before login. I still use Teapot to login to SL regardless since I don't bother much with what SL is doing. I only deal with my own RPG sims in SL now anyway. I haven't surfed around SL regions in over a year and I don't buy anything there any more. I use to have 5 sims in SL up to a year ago and I always spent a lot on content. I just have the 2 sims now and the only money I spend is what I pay to the Land Baron and that is offset by what the players donate or spend in the market.
I have had discussions with my SL role players about their viewers of choice too and most do use Firestorm and some are miffed that they will have to keep two versions - one for SL and another for OS. More of my role players are now grid hopping to follow RPG storylines so switching viewers is an inconvenience to be sure. I have actually been recommending Teapot to them and some are using it. But, that all said,I really don't see why Firestorm team can't use mode switching between SL and OS anyway so the grid manager can stay in and people don't have to have two versions. Of course if they are worried LL will block their viewer then it's understandable but if it had mode switching so the Havoc issue didn't arise then blocking the viewer would just prove the point that LL's action is just as much about blocking Opensim as complying with the Havoc license.
Anyway, I will wait and see what the Opensim flavor of Firestorm is like and how it performs before making a commitment to using it on a regular basis.