I am a longtime Tacx Flow user. I used one for about three years before I got my SRM, and since then I've used the Flow for another three winters with the SRM. Based on my experience, I can say that the Flow is not "accurate" to the extent that it tends to over-estimate my power output; this effect is exaggerated as drum/rear wheel speed increases (ex. the Flow is closer to my SRM readings at 30kph than it is at 45kph). This is unfortunate because the Flow feels much smoother and more realistic at higher speeds.
But, the Flow is still perfectly suitable for training because it is CONSISTENT. So, even though "350W" on the Flow might not really be 350W, it will still require the same effort to produce those "350W" again on another day under similar conditions. So, as long as you keep tire pressure and drum tension consistent, the Flow will tell you whether your relative efforts are harder/easier and thus allow you to effectively structure your training. The same applies if you want to compare performances from ramp tests every few weeks.
I've never tried the Lemond Revolution, but I can say the Flow is the smoothest rearwheel trainer I've tried (and I've tried many). Maybe the Revolution is significantly more lifelike; on the other hand, I gather the Flow is much, much quieter and is definitely smaller. If you get one, it is definitely worth using the trainer-specific tire (much quieter and smoother, in addition to saving your real tires for the road).
Also, I have ridden Computrainers before, and I thought they felt absolutely miserable. Maybe they're more accurate, though--would have to ask someone else. I wouldn't want to do my long sessions on one, though, that's for sure.
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