I've used glass heaters most of my [20+] years in the aquarium hobby --- various makes, models, sizes, wattages.... Not having used this brand before, I was a little skeptical, but it fit the size to wattage ratio that I needed for a particular aquarium build. First thing that stuck out was the length --- the product specifications say 13", but they don't account for the temp. control knob on top which adds another half inch or so. This is quite a bit bigger than most 100 watt heaters I've used before (I have a 300 watt which is half the length!) Next thing I noticed, after setting it in position is that the holder just isn't designed to secure a heater this length to the aquarium glass -- the suction cups just aren't strong enough, nor does the holder cover enough of the heater's length to secure it. So, I replaced the plastic holder that came with it with three clip-with-suction cup holders from other heaters I'd had in the past whose diameter was a perfect match. THOSE hold the heater in place long-term, the holder that comes with it had to be re-secured after a few hours.Now for the real issue..... I set the temp. & placed the heater in the tank and let it sit for more than 24-hrs. before plugging it in.... Didn't take long to realize that it was running much longer than it should've been in water that was at room temp. to begin with. Checking the in-aquarium thermometer told me the heater was running well above its set temp. --- I had it set to 74˙F and it got up to 78˙F..... In the end, I had to set it to 68˙F to maintain a min. temp. of 72˙F. So assume that this heater isn't accurate from the start. HOWEVER, once you figure out its sweet spot, it maintains temp. flawlessly. But, again, note that it runs higher than set --- have to be particularly careful if you have lighting systems that radiate heat at the water surface as you could end up with a tragedy on your hands. I would NOT recommend this heater where temp. accuracy is paramount & is must be maintained in a very small range --- like saltwater systems.While I wouldn't say I don't recommend this heater at all, I will say that it's not for beginning aquarists to be messing with and those with sensitive aquatic systems (like live reefs) should avoid this heater. If, however, like me, you're after a heater to maintain a minimum temp. in a system where natural fluctuations within a 5-10˙F range of temperatures isn't a problem (or actually encouraged), this heater will do that reliably once you find the setting that gets your baseline temp. right. Whatever the use in your case, make sure to pair this with a reliable in-aquarium thermometer at least until you get its temp. off-set figured out.To summarize:• Long heater, not suited for small aquariums• Inaccurate -- mine is off by 4˙F (to maintain 72˙F, I had to set it at 68˙F)• Not suited for beginning aquarists (due to accuracy issue noted)• Not suited for sensitive and/or unforgiving systems (due to accuracy issue, again)• Isn't safe to use without pairing with a good in-aquarium thermometer• Suction cup holder inadequate to hold the heater in place• Once established, it maintains temp. flawlessly