I bought my first Synology NAS in 2006 -- the CS-406. The box was small, quiet, and better than the PC I was using as a do-it-myself file server. Speed was good and the product was well-designed. Much has changed in Synology products over the past six years, some for the good and some for the bad. The hardware is still solid and performance is still great, but I'm not sure I would recommend this NAS to a nontechnical business user. Other products in this class make setup and ongoing backup much easier.
Unlike some NAS manufacturers, Synology clearly breaks out the different models into different market segments, from workgroup and home use to large business. The DiskStation DS1511+ is one of seven models in the Small and Medium Business segment. Surprising as it may seem, the model numbers do mean something: The last two digits refer to the year of release, while the first two digits refer to the number of drive bays (the "+" means performance). Fifteen drive bays? The DS1511+ unit itself has only five drive slots. Where do the other ten drives come from?
Well, if you buy two of Synology's DX510 expansion chassis (each of which holds five drives) and connect them to the DS1511+, you now have fifteen drives. Synology marketing says this makes the DS1511+ the "most scalable in its class." I don't disagree with this claim, but I have always found this type of expansion silly. I'd rather buy a second NAS box than two additional chassis, which are as big as the NAS itself and cost about $500 each.
Synology sent the DS1511+ with five 2TB drives (Seagate Constellation ES). On the Synology website, you'll find a full compatibility list of drives (including SSDs) supported in this and other models. At this point, the DS1511+ supports Hitachi and Seagate 3TB drives if you want the biggest hard drive on the market.