Any GPS can be legally used in an aircraft for "situational awareness", meaning you can use it for information but it can't be your sole means of navigation if you're doing something that requires specific equipment (like operating in class A airspace or shooting an approach).
Many generic/cheap GPS devices include an airport database, so even though they're not certified for aviation navigation use, they can provide valuable data. Don't worry too much about multiple antennas. Just put the sucker up on the glareshield and it will work just fine. Flying the T-37, until my squadron bought a couple of handheld aviation-oriented GPS devices (not nav certified but they had full aviation databases including navaids and airports), I'd often borrow a friend's gps. It made for a nice nav backup gadget in case something broke.
One trick we used was to use the gps course to our destination to "cut the corner" when we were flying VOR routes. Because you can't receive most VORs more than 60-90 miles or so depending on altitude and station strength, and our gps wasn't certified for navigation, our only official navigation method was to fly VOR to VOR which often wasn't anywhere near a straight line towards the destination. If we had a GPS however, we could simply ask the controller if we could fly the heading listed on the GPS until we could legally fly direct to our destination. It would go something like "Skull 41 request heading 110 then direct college station". They'd usually say yes, so we'd use the course from the gps and fly that until we could pick up the VOR at our destination. At that point, we'd use the VOR to fly direct until we started our normal arrival procedures. Sometimes we could cut 40 miles or more off of our route by doing this, and that meant more practice approaches at the destination or arriving with more fuel in case we had to divert or go missed approach due to weather or some other reason. When going on instructor continuity training cross country flights, this was a great tool for adding more of a fuel pad when stretching near our max range.
