I was able to cycle my canister filter for my 55g doing a fishless cycle in a 5g bucket in less than a week using SafeStart. I was shocked, because I really expected it to take MUCH longer (more like 3-4 weeks), but it was cycled in only 5 days. I dosed it with ammonia to 4ppm, tossed a 50w heater in the bucket set to 86, put a lid on it, and waited to see nitrites. The nitrite spike happened around day 4 lasted less than 24 hours.. if I hadn't been obsessively checking the water I might have missed it, but nitrites started to show up, then suddenly went off the chart later that day, and then by the next day they were gone, and I had lots of nitrates. It was demolishing ammonia at that point.. I kept adding ammonia and it would all be converted to nitrates by the next day... like magic. I was worried it was too good to be true.
I set up my 55g tank with that filter about 10 days ago (about two weeks after the filter had cycled), I added ammonia twice more to the tank which was gone in 24 hours each time, and I added the first fish last Friday... who are all doing great. Ammonia/Nitrites are still at 0, nitrates are holding at about 5ppm since adding the fish (they were a little higher when I was "feeding" the tank ammonia, but I'm assuming they have dropped some due to being heavily planted).
My opinion is.. it works, but only in certain conditions. I've heard that it doesn't work well with fishless cycles, and either I got insanely lucky, or it did indeed work just fine. I bought mine at Petsmart and looked for the "freshest" bottle they had based on the expiration dates. I wouldn't put fish in a freshly set up tank and trust that it would work... maybe a lightly stocked tank could handle it before ammonia was a big problem, but it can certainly kick start a cycle if you happen to get a "good" bottle.