I have read that eSATA is currently fastest
That's pretty much it. It uses the drive's native interface, and it's faster than pretty much any drive can handle (save for some high-end SSDs), whereas USB2 is quite a bottleneck. The downside is, not that many computers have a eSATA port yet, so having eSATA only might somewhat limit your options. If being able to use it everywhere (lowest common denominator, if you will) isn't much of a concern, then this is a great solution. There are also some direct attached storage enclosures (to use many drives, sometimes in RAID) on a eSATA port which are fairly inexpensive too. Addon controllers with eSATA ports are dirt cheap as well.
USB 2.0 allows for the longest cable but with some drive overheating/unreliability
A lot of USB 2 enclosures plain suck. There's some of decent quality, but the vast majority don't really have much in terms of cooling (they don't expect you to do much with it basically) which also affects reliability quite a bit, and on many enclosures, long transfers tend to slow down dramatically (I've seen some transfers of a few GB start at 30-something MB/s and drop below 5 by the end). With non-native interfaces like USB, there's a "bridge" chip (which translates the requests, if you will) which can also be a bottleneck and overheat if you ask too much of it...
NAS is slow and usually complicated
I wouldn't say it's that complicated but it's by far the slowest option, unless you're willing to pay top dollar -- and at that price point you can usually do something FAR better/faster yourself (FreeNAS, etc). The main point is supposed to be ease of setup over a "DIY" NAS solution (again, like FreeNAS on your own box). Life's too short to be waiting after most of these devices.
USB 3.0 (when available) will potentially be faster than eSATA
USB 3 will probably end up "killing" eSATA in the long run. It'll be super fast when people have USB3 ports (as much as eSATA for traditional drives basically, as neither is a bottleneck), but it's also backwards compatible with any old USB port so it also works everywhere -- best of both worlds. USB3 controllers are also inexpensive (like eSATA cards). So this is a great option, assuming the enclosure doesn't suck of course... By the time most people have USB 3 ports, I don't think there will be much of a point to eSATA ports anymore (kind of like how we see a lot of firewire ports on the current mobos now that we've mostly moved on). eSATA may still be a good option until that day.
Has anybody found a USB 3.0 solution?
Newegg seems to have a LOT of USB 3 enclosures
. Not all enclosures are equal though, and some brands tend to be alright (vantec) and others consistently suck (bytecc -- they seem to reliably fail).
I found exactly one multi-bay eSATA enclosure on Newegg (AMS VENUS ES5 DS-315SES 3.5" Black eSATA External Enclosure) that I could use with some cheap Western Digital 1TB drives for a RAID 1+0 configuration.
Lots of such solutions are listed in different categories on newegg. Like this unit
for instance ($130 for 4 bays, RAID, good cooling, has eSATA controller, etc). You may even find some open box deals
, or inexpensive solutions
if you already have a eSATA port. Just look around