Not all USB enclosures are built the same.
Chipset controllers make a big difference. In the firewire days, you'd search out Oxford 911 chipsets enclosures because you knew they were the fastest. The same applies for USB enclosures.
Today, I got myself an OYEN DIGITAL Mini Pro USB 3.0 2.5" case. It currently retails for $30.
It has the following:
ASMedia 1051E chipset.
This chipset supports a full SATA III 6GBps drives at full speed. Most of the USB 2 and 3 enclosures support only SATA I & II. Depending on firmware, some 1051E controllers support UAS (USB Attach SCSI) protocol. I don't if that is the case here so I can comment.
So if you plan to find an enclosure/dock, look for one with a ASMedia controllers. The 1051E is currently the best USB 3.0 controller on the market. I've used Gensys Logic GL3310 controller cases and they tend to run 20% slower.
Overall, here are my impressions:
It is a great enclosure reminiscent of older G-Technology aluminum G-Drive.
This one has a black matte aluminum PVD like finish. It is very high quality.
It is surprisingly good and pushes the limites of current USB 3.0 to SSD enclosures.
Using a Samsung 830, I've exceeded the BOT (Bulk Only Transport) bottlenecks going over 260 MB/sec.
I do not know if this firmware has UAS but it is definitely fast.
Most importantly, this case is cheap compared to its nearest competitor at $75, Mercury Elite Pro mini which also has the same ASMedia 1051E controller. (http://eshop.macsales.com/item/Other+World+Computing/MEPMU3ESK/)
The Mercury Elite does have an additional eSATA connection but I don't know if that justifies the $45 premium.
Furthermore, in my experience, eSATA has been capped at 200 MB/seconds. You will need a special eSATA 6Gbps card that supposedly takes more bandwidth but I have not yet seen it to be proven in any tests. Most likely, your standard eSATA port on your existing computer is limited to 1.5-3Gbps.
In short, if you are looking for something around $20-30, definitely look at the Oyen.
Here are some quick speed test using a SSD to test its full potential.
Here is a quick youtube benchmark video. You can see it peaking over 230 MB/sec write and 270 MB/sec reads.