I have two of the StarTech 3.5 inch 4 Drive eSATA USB Multi Bay External Hard Drive Enclosure (SAT3540U2E) and I can recommend them unreservedly - based on my experience. They are easy to set up, very quiet relative to most other equivalent units, and they have a compact footprint. They are also really excellent value for the money and contrary to what others say they are amazingly well constructed given the price.
I use them through the eSata facility using a 4-port expansion card which of course is port-multiplier aware - necessary for these units. They do also of course work through USB but much more slowly.
I have 4 x 2TB discs in one unit and 3 x 2TB + 1 x 1TB in the other. The first unit is used for Time machine backups and the second unit is a backup (a copy) of the first for security purposes. SuperDuper is used to keep the second unit up-to-date. This arrangement permitted me to dispense with my Drobo FS which in concept is a wonderful idea, but in reality, for anything other than basic small scale applications it is a nightmare. I am glad to get rid of it.
For information, I supply the following to help others decide if the StarTech units will be compatible with their systems. I couldn't find this information anywhere on the web and neither SonnetTech support or StarTech support could give me a definitive answer prior to purchase.
I have two of the the StarTech 3.5 inch 4 Drive eSATA USB Multi Bay External Hard Drive Enclosures (SAT3540U2E) and I have been using them successfully attached to a Mac Dual 2GHz PowerPC G5 (MacOS 10.5.8) though a Sonnet Tempo SATA X4P expansion card. Both also work perfectly on my 17" MacBook Pro (MacOS 10.6.8) when connected via a Sonnet Tempo Sata ExpressCard/34. The chipset (EXAR - XRS10L240) on this StarTech unit is thus compatible with Sonnet's excellent although expensive expansion cards.
For information, however, I also know, from experience, that the Sonnet Tempo SATA X4P expansion card does not work with arrays/units which use the PLX/Oxford - OXUFS936QSE chip set - which is used in the StarTech 3.5in 4 Drive eSATA USB FireWire External SATA RAID (S354UFER). I purchased one of these StarTech units in the past. The S354UFER does however work with the Sonnet Tempo Sata ExpressCard/34. Also for the record the S354UFER is an impressive performer with built in Raid. I can recommend the S354UFER without reservation.