I've been riding my recumbent trike (aka The Dorkmobile) for a little over a year and 1,500 miles now, and even convinced my brother to buy one.
Lately I've noticed more and more recumbents, including trikes, on the roads and trails. So in case you're thinking of riding one, here are some of my experiences so far.
Riding a recumbent trike is surprisingly different from a traditional bike, and takes some getting used to. You are low to the ground, don't lean into turns the same way, and use different muscles to pedal.
I like to describe it as pedaling a lawn chair, though the sensation of motion is more like a go-cart than anything else. If you're transitioning from a traditional bike, expect it to feel weird for the first hundred miles or so. You will have to learn to brake evenly with both hands (to avoid brake steer), and going really fast (30+MPH) feels unstable at first even though it isn't.
The biggest advantage of the trike is comfort. I can ride all day with no ill effects other than a little muscle soreness; on a traditional bike, my shoulders, wrists, and butt would be killing me after just a few miles.
I also find the trike a lot more fun than a regular bike. The low-to-the-ground position gives a great impression of speed, and the machine is unusual enough to get regular comments (and compliments) on the trail.
Between the comfort and the fun, I find that I'm riding a lot more on the trike than I ever rode my bike. I never tracked my bike riding very closely, but the 1,500 trike miles in the past year is probably close to my entire lifetime bike miles.
I didn't appreciate this until after a thousand miles or so, but adding a third wheel at least doubles the mechanical complexity of a trike over a traditional bike. Two front wheels means that steering is accomplished through a bunch of mechanical linkages; and the very long chainline means extra gears and the opportunity for the chain to oscillate wildly if it isn't tensioned properly.
The trike also has a much bigger footprint than a bike, which makes it harder to store and transport. There are few trike racks made for cars, so I've usually wound up strapping it to the roof (which works really well as long as you only need to carry one trike). It takes up the space of two or three bicycles in the garage.
The third wheel also adds some drag, so the trike will not be quite as fast as a bicycle. If you care about speed, this is probably not for you.
A trike has many of the same safety considerations as a bicycle, but also some differences. Just as with a bike, when riding on roads visibility to cars is a really big deal. Big flags and lots of flashing lights are a good idea. I have a flashing light mounted on my flag pole, which puts it near eye level for drivers. I've not had much difficult being seen.
Another issue is that with the low posture of a trike, sometimes seeing around cars is a problem--for example, when I'm stopped at an intersection and a car pulls up next to me a little too far. On a traditional bike, I would be able to see over the car's hood, but on my trike my eye level is about at the top of the car's wheel. This isn't normally a safety issue, since the car usually knows I'm there, but it is annoying because I can't go until the car stop blocking my view.
The trike does have a huge safety advantage when it comes to stability. Three wheels and a low center of gravity means that a trike is very hard to flip, whereas a bike can wipeout on even a small patch of slippery or loose ground. If you do flip, there isn't very far to fall. I've flipped my trike twice, and both were essentially non-events: get up, brush myself off, and continue.
(In case you're wondering, the recipe for flipping a trike is to turn from a road onto a sidewalk carelessly. If you take the turn too fast and cut the inside of the corner, the inside front wheel will bump up on the curb and your momentum will flip you right onto your side.)
A recumbent trike is not for everyone. It is more expensive and someone more maintenance-prone, and also a little slower than a bike.
But for me, it has been worth it. I ride a lot more often, and a lot further, than I ever did on a bike, just because it is so much fun. If you're thinking about a trike, I suggest that you give it a good long test ride. If you're grinning so much that you have to pick the bugs out of your teeth afterwards, then there's your answer.