Excal 1000, Excal II, Sovereign GT, CZ-20, Tiger Shark, Tejon, GTI 1500, Surfmaster Pulse, CZ6a, DFX, AT PRO, Fisher 1235, Surf PI Pro, 1280-X, many more because I enjoy learning them. New Garrett Ca
The Impulse is an older PI from Fisher. The newer ones by Garrett, Whites and Tesoro are much better in my experience and don't eat batteries as fast. Though the Impulse was better in it's day on battery usage compared to the rest of the PI's made back then.
The Impulse also loves to find iron and will find it deep. In fresh water (you didn't say where you were wanting to use it, as that is important) bobby pins can last decades so you will dig deep holes for them. Time you could better spend searching for the good stuff.
Its a good salt beach machine with the problem that you need three hands to use it (one for scoop, another to hold the machine and the third to keep it tuned).
Always thought the battery life was excellent from a handfull of AA's as Dave Johnson found a way to scavage back current from the coil in some way (80 hours ?). Depth wise as good or better than the Deepstar 2.
If your going to be using one at your home address I would take on board what Sandman says about rubbish in fresh water and pick a machine with discrimination.