Jake,
Check out the thread posted at the top, I posted some results, you can compare your chill times to what I got and it will give you a good idea.
I seriously have to almost shut off my cold water (55 deg) when running through or it over-chills the wort, amazing.
The downside to them is that the wort runs through the middle, so if you have a bunch of material running through, it *could* get plugged or trapped in there. Never had it happen to mine, and I always start by filling the line with boiling water from my HLT and let it set for a few minutes to ensure what, if anything, might be left in there from the last chilling is at least not going to infect anything. I then finish off by running some hot water through again after the chilling to flush it.
Of course, with immersion, you never get wort inside it, so the proof it's clean is easier.
JQ