The down speed and temp monitors help accelerate the learning curve IMHO. Getting tuned in to blowback, bend in the diver rods and the action on a thumper rod takes some time - especially for the recreational fisherman. Admittedly - I only fished a few years without one and saw a fairly dramatic increase in the # of hookups after adding one. The key a lot of times is repeating the set up that works (lead, depth, angle & speed) and the down speed takes the "guesswork" out of it.
I definitely agree that temp a lot of times isn't all that important IF the fish are there and in feeding mode. A lot of times though finding the right temp or a temp break puts you on active fish.
As far as recommendations are concerned I would highly recommend the Depth Raider w/the Sub Troll second, $$$ is roughly the same. I personally like the large digital readout of the DR and the smaller display and probe vs. the sub-troll. I fish w/both (not on the same boat ) and they are both quality units.
My two cents. DAN