I've had one for ~10 years (Elite Volare Mag Lite) and I will man up and admit that in hindsight, it was a waste of money. It got used for ~3-5 hours during its first few weeks of ownership and has sat mummifying in the garage ever since!
Just to compound the bad investment further, I bought a front wheel riser from PlanetX the other week for a tenner, thinking I would dig the trainer out and use my old singlespeed bike.
Except on the way down to the garage a while ago, it suddenly dawned on me that the singlespeed (Tricross Singlecross) has bolted wheel axles, rather than quick release. The trainer's clamp fits over the bolt fairly snugly, but I have no idea if it will be safe to use with my 90-odd Kg lump sat on it!
If you can set the trainer somewhere where the noise does not drive others crazy while you train, where you can use gadgets/tech to get over the boredom, they can be useful (eg. it's blowing a gale outside this afternoon here).
But it is very different to the sense of freedom I feel when cycling in contact with tarmac and off-road trails.