For me a turbo is likely to be the only way to keep my fitness up over the winter as I don't like to cycle in the dark (puts going after work out) and the weekends tend to get taken up with family / football / golf.
The good thing about them is that it is your own bike you use (unlike an exercise bike) and you can use the gears to determine your workout programme - I tend to start off on easy gears and work my way through, unbelievably if you are watching footie the time flies!!
You can also buy what they call a riser block http://www.tredz.co.uk/.CycleOps-Riser-Block_4276.htm , what this does is allow you to adjust the height your front wheel is off the ground to stimulate hill climbing.
Link to my turbo trainer - http://www.tredz.co.uk/.CycleOps-Mag-Trainer-With-Shifter_4272.htm