Tweaktown's review shows good things, especially for the $400 price, and it really depends of how many applications you use, me personally i would end up using more than that, but a high capacity HDD can fix that
havent used one personally yet, but will be including one in my gaming build ill be making soon. if you have limitless funds, get as big of a SSD as you need if you are going to be using it for EVERY thing. There's benchmarks out there as to which SSD's have the fastest speeds. Normally, most people will recommend a 128GB. Big enough to put on your OS(s), and to install your main programs/games that you are going to play. I personally will be going with a 256 more than likely, and getting a 1 TB HDD for everything else.
From what i've heard though, its just like night and day difference. Youtube some video comparisons and you'll see how much faster things are