Trikes are a whole other world compared to their two wheeled counterparts.
First off, most cycle riders cannot ride a trike !
Strange but true, the first time they ride one, they creep around at one mile per hour and at the first turn they fall off.
Trike riders who also ride two wheels also fall off the two wheelers, but the reason is that on a trike, when you stop at lights and junctions, you don’t need to put your foot down, and so the trike to two wheel converted rider forgets that on two wheels you must do just that, and…over he goes!!!
One issues that you may not have considered is the camber of the road, bicycles stay upright unless leaning trhough a corner, even if the road has a bad camber, but a trike cannot do this.
If you are considering putting lots of miles on such a machine, and you already have a bad back, you might want to do a little physical preparation.
You can’t squeeze through quite as small gaps, as a bicycle, but actualll in practice there is not much in it.
Trikes are slower, but they can carry much more and can go far slower when riding up a hill, so you can use the extreme low gears far better than a bicycle can.
This is just as well too, because trying to push a trike uphill is a real pain due the back wheel catching your leg, make sure you get one with lots of very low gears.
Brakes are always fun on these things, some have a double brake on the front wheel alone, the problem with having back wheel brakes is that if you get uneven grip, it’ll spin you around so quick you just do not have the time to react.
Others have a back brake attatched to the rear axle on either a drum(heavy not not cool at all) or a disc(fairly cool), but the best arrangment I have seen is a back axles that allows drive to go to either wheel or both, a bit like a differantial and also has the disc brake attatched so that it affects both wheels(extremely cool and will draw an audience)(most trikes only drive one back wheel)
One significant benefit of a trike is that you don’t need to remove the back wheel from the machine to repair a puncture, you just lever your tyre away from the rim, check for protruding objects and replace the tube, keep the old tube and repair it at your leisure.
I have seen these at fabulous prices, several thousand $, especially the tandem trikes, which are ultra ultra cool.
I don’t advise you to get a conversion kit, unless you are prepared to spend a great deal on it, as the cheaper ones are usually a bit of an ugly lash up and they don’t tend to have many of the options available like two wheel rear drive.
Of course you could always go for a recumbant trike, they are pretty cool, you can pay rather a lot for them and I personally would not recommend one in busy commuter traffic, they place you just out of the view of drivers.
Here’s some more info,
http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3507
It has to be said that this is possibly one of the best cycling sites on the entire net.
Ask any questions you might have on their forums.
Here is a link to a frame builder that is renown in the UK for their superb handbuilt trike frames (at a price)
http://www.longstaffcycles.co.uk/
I think trikes are great, even though I don’t ride one.