I used both Rainbow and Emerald viewers until Emerald went kaput last year for reasons explained in this blog entry. From the ashes of the Emerald Viewer arose Phoenix, which had most if not all of the functionality and user-friendliness of Emerald, and a great graphics engine. It has had its bugs, such as objects (both local and attached) suddenly de-rendering with right-clicking only working to re-render them sporadically, and conference and group chat windows not showing who else is participating, but overall the user interface was and always will be far superior to Linden Labs' Viewer 2 and several third party viewers based on it, like Kirstens. Of course, Linden labs is jealous of third party viewers that give users what they like and want, and they're determined to kill off all Viewer 1-based viewers by breaking the code that allows them to function.
For its part, the Phoenix team decided it was better to suck up to Linden Labs than it is to make users happy, so they've been working long and hard on a viewer that is in most aspects that I can see identical to Linden Labs' Viewer 2, which they've called Firestorm. The viewer is currently in public beta mode. If this weren't bad enough, the Phoenix team apparently thinks that it can dismiss legitimate concerns with the user unfriendliness of Firestorm by insulting its users.
I'm happy to say though that the unhappy folks are only a small minority, however they can be very vocal, loud and discouraging.Yeah, way to alienate your user base. Maybe if you didn't respond to user complaints with such snark as "Viewer 2 is coming, get used to it," you wouldn't be forcing harsher replies. Alas, some people never learn. Oh well. At any rate, with Linden labs forcing everyone to adopt its inferior Viewer 2 software and layout, it looks as though a lot of long term users will find themselves forced to leave Second Life for greener pastures. Fortunately, Singularity's development team has managed to do what the Phoenix development team apparently refuses to do, namely, make a Viewer 2-based viewer that actually has the superior Viewer 1 layout and controls. I heard through the user support group in-world that they are working on getting the mesh import feature, and that they are very close to succeeding in bringing it in. No date was given, which is understandable considering that anything can happen which might delay release of an updated viewer. But at least Singularity seems to be trying to make its users happy. And they do seem to listen to people's concerns with a much friendlier attitude.
In the meantime, I've installed the new Rainbow Viewer update and it is working splendidly so far. It is anyone's guess as to exactly how long it will remain functional, however. As I've written, Linden Labs is working diligently to break Viewer 1-based viewers for Second Life. Rainbow will probably end up joining Phoenix and all the other better viewers out there on the Viewer 1 trash heap. But until that happens, I'll enjoy it and Singularity until they are no more or until I am no more. I'll keep Firestorm around just in case its developers wise up, but chances are I won't be using it much, if at all.
UPDATE (10:09 PM EDT/7:10 PM SLT): Calisto Llewellin has brought it to my attention that Singularity is indeed based on Viewer 1. My apologies for giving the wrong information.