I recently dealt with a similar question...I wanted to add a NAS and based on my needs/desires/$$, I was left with deciding between a BYO using FreeNAS, TeraStation and the TeraStation Pro. Since the BYO was free, I started with trying that out. I have an old 500mhz PIII with 192mb ram and twin 6GB hard drives (I was going to get new, larger drives but I tried it with what I had laying around). I tell you what...that FreeNAS is good.....really good! I used v0.66 and it has a lot of functionality, easy to setup, fast, etc... But, what scared me away is that it is still very new (haven't even gotten to v1.0 yet) and I was afraid of losing my data in trying to save a few bucks (to me, my data is worth a LOT). Plus, I am not a programmer and I don't have much experience with FreeBSD (the core OS of FreeNAS). So then my decision was left between the 2 TeraStations (Pro and Regular).
Ultimately, I ended up with the regular TeraStation (the one with IDE drives) and I am VERY pleased with it. It is small, QUIET, easy to setup and fast (or at least, fast enough for my needs). I too saw reviews stating that these units were slow (one guy mentioned it taking 15 seconds to open 400k sized pdf's). As I am sure you are aware, performance of any device/component is dependent on several factors. All I know is that I use my TeraStation as a file server (for 3 different PC's) and as a backup location for those machines as well and I have no complaints concerning throughput (my main PC has a gigabit ethernet card (Intel Pro 1000MT) while the other 2 are using 10/100's and being routed by a DGL-4300). For example, I can completely open an 85MB pdf in about 1-2 seconds (the pdf shows up almost instantly but if you watch the network traffic it takes a total of 1-2 seconds). So, is that slow??? Not for me it isn't.
If you compare the Pro vs the Regular TeraStation, it looks like the Pro uses SATA drives and a faster processor so you would think that it might have better performance than my regular version??? Anyways, that is my opinion/experience....good luck with what you decide!