I just purchased the Mustek ScanExpress A3 USB 1200 Pro, and since this posting seemed to have old information I thought I'd contribute what I've found out so far. Below is what I posted to Amazon about this scanner, in general it works on OS X 10.5.6 in both Photoshop CS2 and CS4, but the driver is extremely flaky and it can only scan up to A3 (11.68in. x 16.52in.)
This review is for the Mustek ScanExpress A3 USB 1200 Pro. First off I'm using OS X 10.5.6 (Intel iMac) with all the latest patches. I also use Photoshop CS2, but for my tests I used Image Capture, which is an application supplied by Apple. (Photoshop CS2 is also working for me.) I have the latest driver from Mustek's website, which is from Apr. 15, 2008 (the website says 2/20/2009, but that was when the file was compressed.)
I initially had problems with not being able to scan at the A3 size with a resolution over 150 dpi. If I tried anything over that resolution at A3 (close to tabloid 11x17, I'll talk about that later) the scanning would stop and the application would crash. Quite frustrating! However, I found the solution to this issue was to give my user account Admin privileges, I'm security minded and don't like that idea, however at the moment I'm willing to work with that issue. (I think this could be a reason for many people not experiencing any issues with their scanner, if you always have Admin access you probably won't have this problem.) Also, if you are experiencing problems create a new admin user account and try it with that account, the driver for this scanner seems to be very flaky.
The main reason I bought this scanner was to be able to scan 11x17 pages, I know A3 is slightly different, a little wider and slightly shorter. Well the bed of this scanner accepts an 11x17 page with no problem, except you can only scan up to 11.68 in. x 16.52 in. That's right! When they say it's an A3 scanner, they mean it's an A3 scanner! If you work at 11x17 and cover the whole page you are going to be stuck doing two scans. I doubt that will be an issue for me, but this is something that you must be aware of. Others have given the idea that you would be able to scan a full 11x17 sheet, this is not true on OS X, it may be true under Windows, but that's not what I have so I can't give any advice there.
The quality of scans is acceptable to me, I'm primarily concerned with scanning line art.
The driver software seems limited, my main issue is with setting the scan area, you are not able to type in a size you have to drag the selection area. And, of course, the whole issue of not being able to get a full scan at 11x17, when the bed of the scanner will accept that size with no problem.
Finally about Photoshop CS4, I downloaded the trial just to try it out. It does work. However, you must load the Twain driver for Photoshop, in CS4 it isn't loaded by default. If you downloaded the install you must also download the Goodies package, otherwise it will be on the Install disk under the Goodies directory. This is the reason people say Photoshop CS4 doesn't recognize the scanner. When in doubt use Apple's Image Capture application to test the scanner out.
In general for the price it's acceptable, but the driver is flaky, in fact, after I removed Photoshop CS4 the driver quit working properly for me again under my user account, so I have to use it under my main admin account.
Price: 5 stars (Can't be beat!)
Driver: 1 star (Can't be fixed!)
Functionality: 3 stars (This is because it can't scan 11x17.)