I use a ZooMed under my 29 gallon and you're correct, without the retrostat(sp?) it gets too hot, around 100 F. with the temp control it stays at a cozy 85 & & can also turn on my over head red heat lamp so I can watch them at night and just turn down the thermostat and still maintain the 85 degrees. I think they are great for glass aquariums.
BUT I also use human heating pads for the sterilite boxes I keep my amels in for several reasons. 1) ZooMed's UTH's are NOT recommended for use on plastic enclosures which obviously Serilite containers are. 2) They don't have the option of sticking to the bottom of the container so I can move them around as necessary. 3) On the low setting on the human heating pads it maintains a comfy 85 degrees without the thermostat if you need it slightly hotter for those big meals just put it up to medium and monitor the heat so it doesn't get too hot (I only use med when it's rather cold in my house and/or they just had good size meal and only for a few hours at a time. 4) I can use the human heating pads on sandwiched between the plastic tubs and the carpet something again that is recommend that you NOT do with the UTH's. 5) They are a lot less expensive then UTH's and I can use them on ME too 
On the down side to the human heating pads. You have to make sure you don't buy the kind that have an auto shut off after two hours (I've seen them as low as 30 min). It doesn't make for consistant heat which could pose a problem especially after a meal and it would suck to have to turn it back on every two hours. Another minor inconvience is my cats also LOVE the heating pads and have actually nudged my tubs off of the heating pads so that they could curl up on them. I check the heating pads on a daily basis (usually twice a day) to make sure they are still warm and the snakes tubs are still on them (see cat issue above). I can't complain. Both the UTH's and the human heating pads work great for the applications I am using them for. It depends on what and where you want to use them.
Hope this helps.
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Chantel a.k.a. sillygirl
"I came, I saw, I adopted"
2.4.0 green anoles (Stumpy (M)), and the rest remain nameless.
2.2.0 green tree frogs (Romeo & Juliet, Bonnie & Clyde)
0.0.3 D. tinctorius (Cobalt froglets)
0.0.1 Anerythristic 'B' (Charcoal)corn snake (Popcorn)
0.0.1 Snow Corn (KandyKorn)
0.2 Amel Corns (KornSilk & KornMuffin)
0.0.1 Giant African Millipede
2.4 Domestic house cats (Bad Boy (m), Bart(m),Sasha(F),Little Bit(F),Spirit(F)
1.0 Rottweiler (Jake)
1.2 Rats (new additions with no names yet)
1.0 Husband (David, who loves all my critters too)