Even though the SATA Express isn’t meant to be a long term architectural improvement over simple SATA or just PCIe drive interfaces, it is a step to help get more speed out of existing flash based SSDs. The lower cost is what drive manufacturers are attempting to address and attract chronic upgraders to adopt the new bus bridging technology. It’s not quite got the same design or longer term speed gains in the newer M.2 SATA spec, so be forewarned SATA Express may fall by the wayside. This is especially true if people understand the tech specs better and see through the marketing language aimed at the home brew computer DIY types. Those folks are very cost sensitive and unwilling to purchase based on specs alone, much less long term viability of a particular drive interface technology.
So if you are somewhat baffled why SSDs seem to max out at ~500MB/s read and write speeds, rest assured SATA Express might help. It’s cheap enough, but just be extra careful that both your drive and motherboard fully support the new interface at the BIOS level. That way you’ll see the speed gains you thought you would get by trading up. Longer term you might actually want to take a closer look at the most recent versions of the NVMe interface (PCI 3.0 with up to 8 PCI lanes being used simultaneously). NVMe is likely to be the longer term winner for SSD based interfaces. But the costs to date put it in the Data Center class purchasing area, not necessarily in the Desktop class. However Intel is releasing product with NVMe interfaces later this year with prices possibly as low ~$700 but the premium per Gigabyte of storage is still pretty high. Always use your best judgement and look at how long you plan on using the computer as is. If you know you’re going to be swapping at regular intervals, I say fine choose SATA Express today. Likely you will jump at the right time for the next best I/O drive interface as they hit the market. However if you’re thinking you’re going to stand and hold onto the current desktop as long as possible, take a long hard look at M.2 and the NVMe products coming out shortly.