This has become a serious question lately, what with the Emerald splash screen set up to run up bandwidth on a targeted site (are there really enough Emerald users--or even SL users, if all clients did that--for that to constitute a DDoS attack?), and Emerald grabbing a pathname including one's home directory name--though if you're paranoid about that, do you ever post SL client debugging output, or have your client set up to automatically send in debugging information when it crashes?: It's laden with absolute pathnames, too.)
Some claim that quite a few people use--or now, perhaps used--Emerald. (I hope the Emerald developers clean up their act quickly.) Some cite the use of third-party SL clients that preserve the 1.x UI as an indication that many SL residents are voting with their feet against the 2.x client.
So, which SL client to use? KirstenLee Cinquetti's client is very good if you want the graphics bells and whistles enabled, but lately I find it barely usable. After running for a few minutes, it grinds to an unusable crawl. Moving the window to another screen helps, temporarily, but eventually it crashes... and I find it pulling down 2.6 or 3 GB of RAM, which seems an insane amount--there's got to be a memory leak in there.
Imprudence, OTOH, has a 64-bit version, and with graphics cranked up as far as I can (I'm not sure where the "global illumination" knob is for it), goes at a decent pace... but it has a very nasty shadow-related bug, namely that alpha textures flicker in and out of view randomly--notably if you move your camera POV, but sometimes even when you just sit there. (I can see my eyes with shadows enabled, though--yay!)
So, what to do? Necessity seems to be pushing me to Imprudence.
UPDATE: draw distance makes a big difference with Kirsten's client. Cranking it down from 256 to 128 meters cut RAM usage considerably and made it usable for longer before the huge slowdowns occurred. By the time I gave up on the session, it was sucking down 1.3 GB of RAM.