Hi Russell! I'm a trike owner in York. Depending on how big you are (I'm small and so is my trike), you'd be welcome to have a little go sometime.
It would be good to know if you have any specific questions about trikes and triking, but my evangelising speil would be:
As you've realised, balance is easy (I assume that's the 2 wheel issue) - no matter how slow you go, you can't fall over. You're more likely to tip over if you overcook a fast corner, but even that takes a bit of doing - and the different models vary in stability from 'very' to 'rock solid'. There are two basic patterns - tadpole has two wheels at the front, delta has two at the back. Mine is tadpole, and I find it the more stable type, but deltas can be perfectly fine.
Comfort is great, as with any recumbent. Again, different models vary - there are hard shell (bucket) seats, or mesh ones, and various heights and angles of seat - the lower, more reclined ones tend to be racier, the more upright ones more suited to touring.
Speed varies, and of course depends on the fitness of the rider. Trikes do weigh more of course, and like many recumbents tend to be slower uphill (but with a low gear, remember you can go as slow as you like and not fall over), but fast downhill, with great stability at speed.
On the road, you tend to get much more respect on a recumbent - drivers don't know what you are, so they see you properly, rather than dissmissing you as 'just a cyclist'. I think they also oftne think you're disabled - a misaprehension I'm happy with if it means they overtake nicely.
Downsides - yes, there are some. Being low down isn't so great in traffic - not so much for being seen, as for seeing ahead. But it's not impossible, you just adjust your riding style.
Cost - all recumbents tend to be more expensive, because they aren't mass market. But second hand machines can be had, and have usually been well looked after. And many parts are standard, just mounted on a non-standard frame.
Storage: a trike takes up more room than a bike. If you have a garage, it's no bother, and they can always be hung up by the single wheel from a hook to reduce the footprint. Mine lives in a borrowed corner of a lockup garage.
I'd certainly reccomend trying as many as you can to get the right one. Many dealers can offer various models to try. Follow a few of the links here to get an idea of makes and UK dealers:
http://www.velovision.com/links.html#SpecialistdealersUK
So, any more specific questions?
