Buy it. I use mine with prints, keeps my fiber prints consistent summer or winter. I would think that with deep tank development it would work great, except for the fact that most of the time with deep tanks, the temp stays consistent so its easy to adjust the times to match the temp. In my darkroom summer temps are high so it makes printing easier (and faster!).
Seriously, this a great unit. I've done a bit of repair on mine over the 10+ years I've had mine, if I lost the whole darkroom this would be the 1st or 2nd thing I'd replace. I just can't figure out why nobody has either built another unit like it or improved on it, considering the used prices it gets (300-450!). The guys who designed it did it right, I figure mine is about 20-25 years old and runs great, I just wish the temp unit wasn't enclosed in resin so I could replace the phone style cord when that part goes under. If they want less than 300 grab it.