how good is this samsung 840 pro?
It's fairly low power, with power saving features not active (hence being a good mobile SSD), while being quite fast, especially for write-heavy workloads, but Samsung wants way too much money for them. While still among the fastest desktop/notebook SSDs, the difference between OK and really fast is not much, in practice, and which is really
the fastest depends on what you benchmark.
The 840 Pro tops so many charts, FI, because content creation, which it is great at, is benchmarked a lot, and weighted heavily but test suites like PCMark. Apple also uses Samsung as an SSD OEM, so there's cheating or anything, there: Samsung tweaked their SSD for professional Apple users, more or less. Unless you are professionally performing work that may be limited by the SSD, such as video editing, where mere seconds of savings per day could very well add up to some ROI v. a slightly slower SSD, you'll never even notice a difference, nor is it worth paying more for.
http://www.storagereview.com/wd_black_4tb_review_wd4001faex
That's a pretty fast and large HDD.
http://www.storagereview.com/crucial_m500_ssd_review
That's a slightly better than average speed SSD*.
Notice the typical >500x difference in worst-case write time (but up to 3000x), >50x the write operations per second, >100x read operations per second, and the high transfer rates across the board. The different SSDs are
mostly clustered together, and the different HDDs are, too; but each are so far apart it's just comical to compare them to each other. As a desktop or notebook user, any current-generation SSD will simply
remove the disk performance botteleneck. Sure, you have to wait, sometimes. But, when copying GBs of files of varied sizes completes in just seconds, and doesn't significantly slow down the computer while it's going on, you hardly notice. I very much disagree with skimping on CPU, GPU, or RAM to fit an SSD into a budget; but, if that's not a problem, then I'm all for getting the biggest SSD that can fit the remainder of the budget.
* For desktop/notebook use, anyway. The Iometer workload graphs there show a bit more of its strengths, as a cheapie business SSD, like the M4 and 830 before it.