NVMe makes sense only if you are either pushing bandwidth limits (e.g. processing large RAW 14-bit 4K/8K video on a scratch drive) or have hundreds of threads with concurrent I/O operations. In real world, you are barely going to notice any difference between SATA2 and SATA3 SSDs, not to mention M.2 PCIe ones.
With SATA2 you might as well just use a HDD. Of course I'm assuming you actually optimized how your data is layout on the storage medium to take advantage of the sequential read speed of your HDD, SSD or even RAM. Even my HDD usually reaches 160MB/s so a SATA2 connected SSD is only twice as fast at four to six times the cost. Yes SSDs are better at IOPS but an application that heavily depends on IOPS is often a result of poor design.
For real-world reaching speeds of 250MB/s while having significantly reduced latency comparing to HDD as well as read/write IOPS >60k is what gives you the snappy feel of SSDs. If you are unlucky and have e.g. Sandisk G25 SSDs with write IOPS in 10k range, you'd barely notice any difference to a fast HDD. But if you even get an SSD in a USB stick like Sandisk Extreme 64GB with reasonable IOPS, you can install OS X/Linux/Windows there and it would give you that snappy feel. Bandwidth beyond certain threshold is not what is giving you the snappiness. If you take NVMe that reaches 3000MB/s comparing to SATA3 with 550MB/s, booting time reduces by like 1s - would you really want to pay 2x price when that is the only benefit you'd notice? Or starting your app would take 0.63s instead of 0.67s?
Seriously, invest into NVMe if you are video producer (I can't imagine processing my 4K movies on SATA SSD or HDD, even 24fps playback on SATA SSD can't happen in RAW format as it needs >1MB/s) or you do some heavy I/O server stuff. If you don't do any of the above, invest your $ into capacity instead, i.e. given 512GB NVMe vs 1TB M.2 SATA I'd go with 1TB one.
