Top-N-Tilt is an option that adds some hydraulic controls to your 3 point hitch. It consists of two additional hydraulic cylinders that replace parts of the 3ph assembly.
One of the new cylinders ("TOP") replaces the 3ph top link. This cylinder allows you to hydraulically lengthen and shorten the top link from the seat, which effectively allows you to tilt the implement front-to-back.
The other new cylinder ("TILT") replaces the lift arm on the right side. This one allows you to adjust the lift arm length hydraulically, allowing you to raise and lower the right side of the implement, tilting it right-to-left
Top-N-Tilt lets you do things from the drivers seat that you would otherwise have to do by stopping the tractor, getting off, and adjusting the links manually, sometimes many times a day. I can tilt a box blade left-to-right to give a crown to a road or to scrape along an incline at an angle different from the tractor. I can back up to a hill and drop a rotary cutter on the down slope side or raise the rotary cutter to cut an uphill slope. When attaching implements you can back up to them and adjust the 3ph from the drivers seat to line up all the links, even when the tractor and/or implement aren't on level ground. There is no end to the fun you can have with TnT.
Cost-Benefit? How do you rate satisfaction and ease of use? It would add somewhere around $1500-2000 to the price of a JD TEN series machine installed by the dealer. The TnT kit is around $750, plus you need two additional SCVs and a field diverter kit. The field diverter kit allows you to flip a switch and use the loader joystick to control the TnT cylinders (assuming you already have a loader).
Like one person said here on TBN (paraphrasing) don't ever use a tractor with TnT if you don't intend on buying it, because once you do you will never be satisfied without it.
I have it, and I wouldn't want to be without it. Well worth the money if you have the budget for it.