I'm running in the latest (available) development version of the viewer at the moment, and right now I am spending rather longer than I would like staring at a seized-up screen and/or the SL crash logger.
There always seems to be a trade-off between speed and stability when it comes to the viewers, and this current version is leaning rather heavily towards the speed side of the equation. While it's working, I'm getting (what I consider) reasonable frame rates, even on my clapped-out computer. While it's working... but rather often, it isn't.
Of course, some people may be asking, why am I using the LL viewer at all, why have I not "upgraded" to a TPV...? Well, there are several reasons, some of them better than others. One of the less good ones is simply reverse psychology, in that I have long since had it up to here with people telling me how great Phoenix/Firestorm are, and I can't help feeling that if I used those, I would wind up joining some sort of cult. Perverse, perhaps, but there we are.
On a more serious note, sticking with the V3 development line seems to me my best bet of keeping up to speed with innovations. Firstly, although the viewer code is open, the server-side code is not, and so the official viewer is likely to be the first one to synergize with new developments on the server side. Secondly, the TPVs are, in some ways, predicated on not innovating - specifically, they are sticking with the V1.23 style interface that so many people (but not me) know and love. So innovations from the TPV side tend to be what I consider bolt-on bells and whistles, and thus stuff I can happily live without (especially on a machine which has, as it were, limited space for bells and whistles). The useful or desirable innovations tend to make it into the official viewer sooner or later; sometimes, indeed, in a better form than the TPV devs come up with. Compare and contrast Emerald's old extra-attachment-points system, for example, with the flexible system we have now, that allows you to add multiple attachments on one point, and lets you pick which points you use. I know which method I prefer there!
I will also be heretical, in a small way, by saying that, unless you've spent a lot of SL time getting used to it, the V1.23 interface is not actually all that good. I started in V1.23, although V2 was already available at the time... and I switched over to V2 with almost a sigh of relief. I find the old-style screens cluttered, the menu bars over-populated and poorly laid out, and I never once squelched to the bottom of one of those pie menus without muttering "Why can't they put the options that aren't greyed out at the top, dammit?" And some of the TPVs, frankly, compound the problem with even more clutter, with some truly appalling font choices (any font where I can't tell the difference between a lowercase L and an exclamation mark is a bad font), and with default colour schemes that make my eyes scream for mercy. Meanwhile, by now, all the real defects of the V2 interface have been addressed, and V3 is a good, sound, and highly customizable design.
But, then, people do get used to things - I have noticed this a lot with Open Source software, where the user base overlaps with the developer base and so development is geared more towards what people are comfortable with. I don't think this is necessarily a bad thing - in many cases, it's a good thing - but it does seem strange, sometimes, to be looking at software products which, however sophisticated they may be "under the hood", look as if they should have a "Best Viewed With Netscape Navigator" decal on their interfaces. As a largely Microsoft-based programmer myself, of course, I have had to get used to the idea that my UI will be completely revamped every eighteen months or so, and I will just have to suck that up and relearn everything. So maybe I am less likely to become attached to a particular sort of viewer! (I may be the only person who kind of misses the V2 sidebar - it did hoover up a lot of my on-screen clutter into one place! But I'm not losing any sleep over its demise.)
So, anyway. I'm sticking with V3, and hoping to stay up to date with changes in SL. But I could do without all the crashes, thank you very much!