Tetra safe start is similar to the product I used, Dr. Tims Nitryfing Bacteria. Neither product causes any harm, but it really doesn't speed it up either. I know several people who have used Tetra safe start in both fish in and fish less cycling, it's a 50/50 shot, same with the Dr Tims. Neither product needs to be refrigerated, however, they are still temperature sensitive, too hot or too cold will kill the bacteria. Both products are difficult to find in stores, therefor generally they need to be purchased on-line, and if it is not shipped properly (referring to temperature stability here) it won't work, and even if it is shipped properly, what was the temp in the warehouse etc. Just too many variables to ensure you get live product. That is why a lot of stores don't carry it. I have read the forums that swear by it as well, but in my experience of knowing people who have used one or both of these products and my experience with Dr. Tims, don't waste your money. Dr. Tims possibly shaved off a day or two with my cycle but it still took 69 days for the 12 gallon to fully cycle, the 5 gallon is now 2 day away from completing the qualifying week, and the 55 gallon is about 5 days away. So bottom line - 69 days for the 12 gallon, 101 days (62 fish less, 39 fish in) will be the total for the 5 gallon, and 40 days for the 55 gallon (and it had a cycled filter, but borrowed some for the other 2 tanks so put the 55 gallon into a cycle).
Try it if you want, Dr. Tims is about $30 with shipping for 2 oz, Safe Start runs about the same price, possibly a little less. It certainly won't hurt anything, and hey, maybe you will be in the lucky few that it works for, but don't expect miracles.