The M550 series is available in mSATA, M.2 and of course 2.5-inch form factors. Capacities range from 128GB to 512GB in all three form factors, but there's a 1TB 2.5-inch drive, too. M550 drives are based on 20nm MLC, courtesy of Micron. The drives feature 512MB of LPDDR2 cache, but the 1TB version ended up with 1GB, which is pretty rare even today.
The controller of choice is Marvell's 88SS9189 and most SKUs can read and write at 550MB/s, with 75k to 95k IOPS depending on the SKU. However, the 128GB units end up somewhat slower, 550MB/s read and 350MB/s write.
The old M500 series usually peaked at 500MB/400MB and about 80k IOPS, depending on the capacity.
Some SKUs are already available in Europe and the entry-level 128GB 2.5-inch unit will set you back €79. On the whole, the M550 is competitively priced, but there are a few cheaper drives out there (along with pricier ones, of course). All of them are covered by a three-year warranty.