Due to work and family commitments, I use both indoor and outdoor for tri training.

Long rides on the weekends of up to 200km and during the week I do a couple of indoor sessions on a mag-trainer Time Trial training with Lances' coach.

I'd agree with both Miners and Marty - you do need to get both in, and if you can't get out, indoor is better than nothing.

Make sure that you make these efforts count - put on your favourite fast music and go hard in each chorus or find a training DVD. The other thing you can use indoor for is brick sessions - ride hard for twenty minutes then go for a pace run, then repeat until you fall over... or something along these lines :D

My uncle(70+) has to ride a bike for a back pain management problem. He told his doc he was riding an exercise bike and was promptly told that it wasn't at all as good for him. The positioning on the stationary bike is way different to a mobile one. He now rides a road bike 20+k's a day, no matter the weather(he lives on the south coast N.S.W).
If it's all you can do though, I guess it's better than nothing.

kazz.

As above posters have said, anything is better than nothing. That said your bike on a windtrainer is going to be better than your average stationary bike at a gym due to the positioning.

From personal experience I did my first sprint tri last weekend (stepped up from Brooks to Gatorade this year). Initially I was disappointed with my bike time, but then I hadn't been out on the bike at all. I'd only been doing 2 windtrainer sessions a week for a month. I know that with some additional time spent on the road I'll get back to where I was last season. It can be tough on the bike when there's a headwind and those conditions cannot be simulated inside! If you can get out on the road then bonus, but it can be done without.

Best of luck for training and racing!

Now that I have bought a bike. I am looking at buying a wind trainer so I can do a bit of cycling through winter at night and indoors. From my research it looks like there are three types:

1) Wind trainers - use wind resistance, are cheap and noisy
2) Magnetic trainers - less noisy - more expensive
3) Fluid trainers - most expensive and most lifelike

I think I might buy the magnetic type for apx $200-300. Any suggestions, particular brand, things to watch out for etc?

for me half of my bike training is really about being used to my bike, having better riding skills etc, so if i couldn't ride for real then i'd feel that half the training benefit was missing.

... but life is a compromise...

Allisonjc, lets approach this a different way.

How do you structure your training week on the whole?
That is where do you fit your running sessions in and are they on a tread mill or outdoors, swimming sessions and any other training?
You don't need to ride on the road alot depending on your skill and confidence levels on the bike. But i would suggest that at this stage of the year you would be better placed to put one session a week as a minimum to work solely on your bike riding skills. This only need to be a 1hr session where out practice cornering, changing gears reaching and drinking from your bidon, looking around you, changing direction, etc.
Working out on an indoor trainer is beneficial and will make you a fitter rider, but unless you are spending time on rollers or on the road your skill level will remain where it currently is.
If you own a hometrainer then good if you are riding a stationery bike you need to set it up as close to your real bikes position as possible or you will also feel uncomfortable on your real bike a this is the one you will be riding the least.
So let us know your training progrm structure and we might be able to suggest something to help