I've fished using a Fish Hawk hand held temperature gage to find the cold water back in the late '70's. Then I ued a DataMarine surface speedometer for trolling speed. We'd find the temperature break and set up above it, at it and below it as we do now you just didn't know when the temperature changed until you stopped trolling and did a re-check of the water temperature.
I moved up to a Mono Troll from Big Jon in 1990 it was a unit that was like the Fish Hawk 840 where you had a wireless probe attached to the downrigger ball and a Tri-ducer on the transom that picked the signal from the downrigger probe as well as giving you surface speed and temperature. I had the Big Jon unit for 12 years my only complaints were downrigger blow-back because of the probe size and probe replacement price ouch!! If you lost a probe it was $250 plus 2 to 4 weeks delivery time. Now if a probe went bad then it was an exchange and it cost $60. I caught more fish once I knew the speed and temperature at the ball.
In 2007 I bought a Depth Raider. I installed in on an old Cannon Mag 10 (aluminum body). This model Mag 10 has a very slow up and down speed. I haven't had problems with stripping the coating off the cable that a number of guys complain about and I connected the probe permanently to the cable. My only complaint with the Depth Raider is that the display unit will over heat on sunny days because of the units dark gray color. I have taken the advice of others that have experienced the over heating problem and put a white cloth over the display head when it's sunny and warm. I've caught a lot of fish using the Depth Raider for ball speed and temp in conjunction with a Lowrance surface speed and temp gage. I like the constant speed and temperature updates the Depth Raider gives. With the old Fish Hawk unit if the downrigger blow-back was too great the Tri-Ducer wouldn't read the ball speed and temp and the deeper you go with the probe the more you increase the probe blow-back as well and I don't like to use weights that are over 10#'s.
Bottom line is wouldn't go back to fishing without a downrigger speed and temperature unit. If I were buying one now I'd buy the Fish Hawk X-4 there's no coated cable to deal with and they decreased the probe size which should help with the blow-back.