Its always going to be better on the road, as its real. Trainers only approximate road riding, and for anyone who can get outside are an absolute last resort.

However, it can sometimes be easier to do intervals and such on a trainer as you do not have to worry about traffic.

My suggestion would always be to get out on the road whenever you can, but if you absolutely can't, then the trainer is a good substitute.

When on the trainer, don't just go at a constant speed. Do interval sets, hill pyramids e.t.c, to make the time go faster and to reap the benefits.

Brian

I love to be outside on my bike and absolutely hate the trainer.

That being said, the trainer is great for those times when it is just impossible to get out because of weather, time or any of the millions of things trying to wreck havoc on or training schedules.

I also agree with Brian. Do intervals. I just get too bored otherwise.

Josh

Road or Trainer? Something is better than nothing "and if you can't be with the one you want, love the one you're with."

anyone recall that tune?

The road is always better, but the trainer can be a great tool. I'd never do long, steady rides indoors (for sanity's sake first and foremost!), but I also live in South Florida, so I don't have cold winters to deal with. (Just hurricanes - many of them)
The trainer is great for -like Brian and Josh said- intervals and strength work (i.e. hills). It will make you stronger. A lot of times such workouts are difficult to do on the roads if you live in either a metropolitan area (too many stops, traffic considerations), or a very flat area.
That said, there's nothing like actual road experience, and I would definitely try to keep my rides out there as much as possible.
Hope that helps!

No contest - the road is better.
Trainer definitely has its place tho bad weather etc.
Also some form work should only be done on a trainer: i.e. single leg drills.

A dissenting voice. It dpends on your available time, the planned workout for the day, and your own discipline. While I would in general agree that there is ultimately no substitute for the road, some workouts are actually more effective on the trainer. Also if you have limited time available, the trainer may be a better option.
For example steady state intervals (e.g. prolonged time, like 10-15 minutes, at LT threshold) are good to do on a trainer. A key to these is holding your HR right at LT threshold, which you can't always do in traffic.
As for the limited time point, if you've got 45 minutes to an hour on a weekday to train, and that includes set up time, you can hop on a trainer, and with a structured workout, get a good workout. Whereas the same time outside, between getting out on the road, dealing with traffic, stop lights, stop signs, and coasting, you'll get less of a work out than you could have got on the trainer.
Thus I wouldn't worry if 1 or 2 days a week is on the trainer, with longer rides on the road on the weekend. Plan your workout schedule, so on the trainer days you're doing workouts that take advantage of what the trainer is good for.

i've somewhat the same problem as adrian because i live in the city centre where traffic is constant even early mornings and late nights, making it very hard to ride on roads. im not driving yet n i dont own a trainer...so mostly i have to find my way to the nearest coastal area..and ride the same stretch of road abt 12km long back n forth. also, ive just started using clipless pedals so smtimes i stil get kinda nervous abt clipping in n out of pedals, and thus dare not ride on major roads.