any one use this? or something like this? what is it good for/what's its application(s)?

I use the non-electric version: http://vexilar.com/products/index.php?prodNum=104

It is used for figuring out what the temperature is at various depths. It is super useful for cold water fish like Lake Trout (but could be used well for other species).

For example, Lake Trout prefer water between 45-55* F. That doesn't mean they wont venture into warmer water if the food is there, just that they will choose that temperature range if they can find food and oxygen there.

So if you check the water temp and find 55* water 30' down, you have a good starting place to look for Lakers - that depth or shallower. They won't hesitate to come up for a bait, even if it means coming into warmer than preferred water.

It certainly isn't essential but it helps. For dead baiting with ciscos I find where the right temperature range meets bottom structure.

I've never seen one like you have pictured but I also use the non electric one vexilar makes. Its a pretty inexpensive way to help locate the depth at which we start fishing for lake trout.

Thanks for the replies. Are you using in summer or ice fishing? would it work for both?

"...For example, Lake Trout prefer water between 45-55* F...So if you check the water temp and find 55* water 30' down, you have a good starting place to look for Lakers - that depth or shallower...." ???

Is this for ice fishing then?

Open water. Especially in summer, lakes will stratify by temperature. It can be a major aspect to consider when trying to find fish.

Rarely will you find water warmer than 39* F while ice fishing - "warm" water under the ice tends to make things less safe... Generally in ice fishing the water temp varies so little that I don't worry about it. Oxygen, structure, and baitfish are more important concerns then.

I use a Fish Hawk: Fish Hawk
These are absolutely amazing ... but pricey. They read the water temperature (in Celsius or Fahrenheit) and give a read out for every 5 feet of depth up to 300 feet. I will also attach it to my line if I want a fairly accurate reading as to what depth my lure is actually running, for example, when trolling. As someone pointed out earlier, these things don't tell you where the fish are but they give you a good idea where to start looking.