As with all Synology appliances, the DSM software is a real selling point making light work of NAS and iSCSI data sharing as well as providing a platform for a number of useful server-like applications.
The quad-core Alpine AL-514 processor is only 32-bit but appears capable of handling the storage sharing needs of small to medium-sized networks, plus you have the option of adding 10GbE connectivity at a lower price point compared to alternatives such as the Netgear ReadyNAS 716 which features 10GBase-T copper interfaces whether you need them or not.
A couple of things gave cause for concern with the DS2015xs, not least the reliance on a single power supply in a platform potentially able to support up to 120TB of shared storage. Power supply failure is far from common, but it can happen and could be a real issue if using the NAS to support business-critical applications. The processor is another concern as, although fine for most applications, when pushed hard it could be a bottleneck.
And lastly, the desktop format seems a little out of place here. Potential customers may prefer one of Synology's rack-mount alternatives.
Sitting at the crossover point between the SME and the corporate datacentre, the DS2015xs will appeal most to customers on a fixed budget looking for a mix of scalable storage capacity and 10GbE connectivity. Those happy with Gigabit networking may be better off sticking with the DS1815+, while companies with more money to spend can get better value from rack-mount alternatives with faster processors, ECC memory and redundant power options.