Sorry to revive this thread but since I've been asking myself the same questions I figured why not give my 4 cents in this matter...!
I went through the very painful process of contacting Samsung mobile tech support and after talking/emailing about 5 totally clueless agents and being given totally irrelevant answers, I finally learned that the S3 is NOT UHS-1 compliant. Which means that purchasing that Sandisk UHS-1 class 10 cards right now is utterly useless. If the S3 is not compliant then I seriously doubt that any other phone is.
Unfortunately I learned all this after purchasing my 64gb UHS-1 micro SDXC card...so if this can save people a few bucks, then alas my research will not have been in vain.
Now, some interesting facts:
Two Sandisk cards were test in the same card reader (internal, on my PC, purchased before UHS-1 was invented), one being the 64GB Class 10 UHS-1 micro SDXC, and the other being a regular form-factor 8GB Class 10 SDHC. Cards were benchmarked using the latest version of CrystalDiskMark with 5 passes of 50MB of data, twice.
64GB micro SDXC: about 9MB/sec Read(!), between 7 and 8MB/sec write
8GB SDHC: 22MB/sec Read, about 7MB/sec write
It is a known fact that host devices which are not UHS-1 compliant will give lower speeds...but...less than half the read speed of SDHC?!?!? I've seen that card (UHS-1 microSD) tested at 28MB/sec read!!! Does this mean that we would get better performances with the class 6, non-UHS cards???