NAND cells - the type of memory used for SSD's - can only be written to an finite amount of times. With modern drives, this is around 100,000 times for SLC and 10,000 times for 50nm MLC (5,000 for the newer 34nm NAND's/3,000 for the 25nm, in the current drives). So, SLC is better than MLC, which is why most enterprise drives are SLC's (and way pricier). After that, they are still fully readable, but cannot be written to. To remedy this problem, SSD's use a spare pool of NAND cells - usually around 13-25% of the drive's capacity, depending on manufacturer, price-point, etc..., which gets used after the controller senses it can no longer write to a cell. So for a given 256GB SSD, you'll end up with roughly 240GB usuable space, with the rest of it reserved for spare area. Now, if you write around 7GB of data/day to the drive, you use up all the NAND cycles after around 360,000 days for an 50nm drive - call it roughly a thousand years (or ~500 for 34nm and ~300 for 25nm). Chances your system is obsolete before then are pretty good I'd say (Those numbers not my own btw, they are from anandtech.com - in this article in fact, on the OCZ Vertex 3 - who have, in my opinion, one of the best and concise writeups on SSD's I have found. For anyone who wants to know how and why SSD's work I'll recommend these two detailed articles: here and here. While they are somewhat dated, they give an great overview on how SSD's work, and the basics hold true for the newer SSD's as well. The newer reviews of drives expand on those articles)Speedwise, the best bet at the moment with a SATAIII controller are either the OCZ Vertex 3 (from personal experience the 120GB version pushes around 440MB/s write/490MB/s read in a 2011 MacBook Pro (sequential writing/reading, ie. the most optimal test - random, ie. real life performance, will of course be lower), and the 240GB version should be faster still) or the Intel 510 series. The Crucial m4/C400 is the "slowest" of the current generation drives from the reviews I have seen.Mike Ista