The release date is getting pushed back… again… So, they are looking at a beta release then quickly (?) followed by a main release. So, we will probably see a beta viewer released in April. When will the main release appear? No one knows. I am guessing May…
While the FS Viewer has caught up with the SL Viewer code-wise, they can’t release the viewer yet as they are still fighting problems. Some FS testers are finding many more problems with the internal FS QA release than with the SL Viewer. The basic complaints are slow rez, some objects never rez, and there are more attachments dropped on teleport problems.
Some users are finding it works very well. The variation in computers and connections makes for these differences in experience with any given viewer on a specific computer. It is hard to know how many are in which camp, problems or no problems.
I recommend you always have at least two viewers installed. However many you install, at least one of them should be the SL Viewer. It is needed for any testing of problems. If you hit a problem with FS, re-log with Firestorm. If the problem persists, then re-log but, this time with the SL Viewer. This pretty much immediately tells you if it is a viewer issue or eliminates that consideration.