I've been using a Cimarron (round bowl version) for about a year. This is my home's first 1.6-gpf toilet, and it has produced mixed results.
We bought it because of its availability (it was at both Home Depot and Lowe's), reasonable price (just over $200), and strong results in the most recent MaP testing (800 g of solids passed).
Positives: Its flush is powerful, quick, and hardly ever clogs -- I've had to plunge it just once, and that was within the first week of use. Other than cleaning, no maintenance has been needed so far. As I mentioned, it is inexpensive and available at national chain hardware stores.
Negatives: The flush is a bit noisy towards the end. But my biggest complaint is the flush's inadequate cleaning of the bowl: smeared remnants of solid waste are left near the trapway almost half the time. (Note: see update below.)Sometimes I can clear the residue by holding the lever down or flushing a second time, but if that doesn't work then I must use a toilet brush. Cleaning was never a problem with our old toilets, although of course they used much more water as they were ancient, 5-gpf American Standards. The cleaning issues have been significant enough that I decided not to buy another Cimarron.
After the Cimarron experience, we replaced the toilet in our master bath with an elongated Toto Drake. While not perfect, the Drake cleans better than the Cimarron, and given the closeness in price I think the Drake is the better buy.
Update (Jan. 10, 2008): It has now been about two years since we installed the Cimarron, and my opinion of the toilet has improved with time. For reasons unknown to me, the bowl-cleaning performance has been much better recently, and I rarely have had to use the brush after flushing. Perhaps the surface of the bowl was made smoother with repeated flushes.
The flush's power remains strong, as I have experienced no clogs in the past year. I still prefer the Toto Drake -- the Cimarron flushes more loudly and takes longer to refill the tank than the Drake -- but the difference between them is not as large as it once was.