1.- Why did you create the Cool SL Viewer ?
Mainly, out of frustration...
I started compiling my own viewers shortly after the sources of the official viewer were made available by Linden Lab, but it was just to add a few custom features and/or to fix a couple of annoying bugs.
Then LL released the first voice viewer (v18.104.22.168) and the changes they made to the UI were so catastrophic in term of usability and functionality (no possibility to separate the friends and groups lists from the enormous "Communicate" window, for example), that I could not get used to it and produced a patch to revert most of the changes they made.
Things only got worst with the next releases of the official viewer and despite a violent reaction of the community against the new UI, LL did not step back from it.
Beside, since the Open Source community started to publish a lot of patches to fix bugs in the official viewer and since LL proved unable to integrate them in a timely manner, it became possible to create home brew viewers which actually both were stabler and provided a better usability than Linden Lab's. Nicholaz Beresford was already providing such a (damned good) viewer, but only for Windows. I then decided to publish my own home brew viewer for Linux: the Cool SL Viewer was born (its first public release was v1.18.4 and got 39 patches over the official viewer, many from Nicholaz, several from myself and a few from others).
Since then, the Cool SL Viewer strives to maintain a coherent (and pretty constant) UI, implements some new handy features that LL would take months to integrate, plugs all the regressions found in newer viewers, and provides a much stronger stability than LL's own viewer.
With the move to Windlight, many users were left behind and could not use LL's viewers with decent frame rates or without systematic crashes after an hour or so in a crowded place, and this only because their computer was deemed "too old" (read two or more years old) to cope with the new rendering engine. I then decided to keep maintaining the v1.19..0.5 (the last pre-Windlight) viewer for them while also moving on with newer Windlight viewers. That's why I'm now maintaining two different branches of the Cool SL Viewer.
2.- About how many people use it ?
This question is hard to reply...
First, I only got statistics for the Linux downloads (Boy Lane is providing the Windows version, and Hyang Zhao the MacOS-X version: they are perhaps able to tell you for their respective platform).
Second, a single person may download both v1.2x (Windlight) and v1.19 (legacy renderer) and use one or the other, depending on their mood or on the computer they run it on (two downloads, but one user only).
Third, another viewer (Hippo's OpenSim viewer) is now using the full collection of patches from the Cool SL Viewer (actually making it another branch of the Cool SL Viewer), and other users (especially Linux users) download only the sources and patch set instead of the binaries, so to compile the viewer by themselves...
I can therefore only give a very approximate figure. Let's say (for the pre-compiled binaries I provide on my site alone) around 500 users for Linux, probably around 10 times this amount in total. But only Linden Lab could tell you the true figure (thanks to the stats they get and to the "channel" identification of viewers).
That's not a lot compared to the 1 450 000 "logged in last 60 days" regular users of SL, but the number of users has been growing steadily with time, as words of mouth spread around the community (while the "logged in last 60 days" figure is stagnating for over 13 months now: it was 1 500 000 in November 2007... Only LL can pretend that SL is getting more users every day, but facts are cruel and pretty stubborn).
3.- What to you are its biggest advantages over the official viewer or other open source versions?
A pretty constant and very usable UI (I do think it helped also a lot to convince Linden Lab that they could not impose the new Dazzle skin on everyone and had to provide the legacy skin as the default one in their own viewer), a high reactivity to new bug fixes and patches (v22.214.171.124 is at its 45th release with 77 patches, v126.96.36.199 at its 13th release with 54 patches), a set of new useful features (as well as optional and switchable RestrainedLife support), two branches with different renderers to cope with a larger range of CPUs and graphic cards.