They're awesome. Tiger Toys was a small line, only 11 figures and the Gadget Mobile (oddly no Chief Quimby though). To me that was the "main line" of figures. Then there's the Burger King set of 4 from 1991-ish. That was a pretty popular set and I love those figures as much as the Tiger Toys line. A few more sets of 4 from Subway and McDonald's, both McDonald's sets promoting the Inspector Gadget films. The first set, in each meal you get a different gadget, and it builds about a 6 inch Gadget figure. Cool concept, but I passed on it. The second set, promoting the direct to video sequel, were just horrible toys. Then there's the few random PVC figures.
I was born in '86, and in the late 80's there was a 12 inch Gadget doll/figure released by Galoob and Bandai. They were the same figures, one just had better arm articulation. I never knew about it until a few years ago. He had spring loaded legs, extending neck and arms (one shot out like a missile) and numerous gadget accessories (yes one was the helicopter!). He also had a removable trench coat. Its such a fantastic piece.
I lucked out so much a few years ago. Someone put the Tiger Toys Gadget line on ebay, a buy it now of $60. They were mint in package, and the lot was only missing 2, the MAD Agent and Dr. Claw figure (which I got both in the past year). I must have snagged it as soon as he listed it. Sadly, these are now going for quite a lot, I noticed most people put them on ebay with a buy it now usually $30+ if its in the package (and thats only one!). The Galoob/Bendai figure goes for quite a lot as well, usually $90+.
If you'd like to get some Gadget figures but don't want to shell out the cash, I suggest the ones from Burger King. At most you would spend like $20. The 4 are Copter Gadget, Inflated Gadget, Scuba Gadget and Surfer Gadget. Except for Surfer Gadget, they all have extending legs and neck. For figures that came in a kids meal, they do a good job. The best ones in my opinion are Copter and Inflated Gadget.