I really hoped that this will be a quality product, as I have another, older, Vantec NexStar USB enclosure, and I have been very happy with it.
I tried to use this dock with a standard 2.5" hard drive. I connected the drive in the first bay, connected power, the USB cable and powered ON the dock. The HDD light for bay 1 blinked and then stayed dark, as a hard drive was not present at all.
I put the drive into the second bay (powering off the dock, first) ... this time it worked.
When I looked more carefully into the first bay, I noticed that the plastic bottom is not flat - there is a big bump where the USB 3.0 connector is. See the attached photos. This bump prevents the hard drive from seating properly and of course to connect properly.
In my view this is a major flaw. I am not sure how this has passed QC.
The dock is on its way back to Amazon and I need to look for alternative.

Great dock, grabbed one of these when on sale and very happy with it.

I have had the previous USB 2.0/eSATA single bay model for some years, and it has been very useful, so when it came time to upgrade some gear, I looked at the new USB 3.0/eSATA 2 bay dock.

I don't like the white colour (since I'm sure it will yellow with age) but couldn't get black shipped to my region. I hate that everything on Amazon is treated differently depending on where you live, and also, Amazon, please stop suggesting stuff that doesn't ship to my region!

I use it both with USB 3.0 for laptop connectivity, but where it really differentiates itself is using the eSATA connectivity as an external device with my old QNAP NAS, which only has USB 2.0 and eSATA - makes backups much faster!

I have a single-bay version that I've used for years, and I have loved it. When it finally failed, I ordered this dual-bay version. I put in two 3.5" drives, connected the eSATA, and the device kept cycling power when it tried power up the drives. One drive worked fine, but two appears to tax the AC power too much. I connected to USB 3.0 instead, hoping it might provide additional power, but having that additional connection also produced the same behavior.

Perhaps I got a faulty unit, but there's no reason to have dual bays if I can't have two drives active at the same time. Returning.

This is the second NexStar dock that I have owned and I bought it based on my experience with the single bay Vantec NexStar USB 2.0 eSATA Dock NST-D100SU White dock which I bought back in 2012.
The single bay unit with a 1TB WD Black Caviar hard drive has been connected to my desktop computer via eSATA as a backup drive for four years now and has performed flawlessly over that time.
The new dual bay dock is being used with two 2TB WD Black hard drives, one as main storage and the second as a backup drive. This dock is currently connected to my laptop via USB3. All three hard drives are available from both computers via my wired network.
One difference that I have noticed between the eSATA and USB3 connections is the tendency for the units connected via USB3 to go to sleep when not being used, whereas the unit connected via eSATA on the desktop is always instantly available. This problem probably has more to do with the USB settings on the laptop rather than a problem with the dock itself and is something I will investigate further.
At this stage I haven't tried the new unit via the eSATA connection but I wouldn't expect it to perform any differently to the single bay unit.
Apart from the (minor) problem with the USB3 connection both units have performed flawlessly and I would have no hesitation in recommending either dock.

I ordered this mainly as a docking bay with the benefit of having a duplicator as well. Got it in yesterday (010815) and tested it with an existing 2TB WD Black that was formatted as 4K. Drive read the data and 5 minutes into testing the HD locked up and had to power it and then system down. Tested drive with original Thermaltake BlacX single bay and data was fine. Did more research this afternoon and confirmed with Vantec that it is limited to 512k HD partitions. This limits this product hard for me (and would think most tech's). I cannot test it's duplicator yet as I do not trust it with partitions that are set any higher than 512k. I will say Tech Support at Vantec was fast, courteous and informative. Hope this helps future purchasers.