I ended up buying one but I am having second thoughts from some of the write ups that I have read on some turtle and amphibian sites.
That is perfectly understandable. I have turtles and am a member on a turtle forum and have come across that filter before. The reason it gets bad reviews is because it's horrible for turtles! Turtles require a huge tank and constitute a very large bio load. Something that the 501 is in no way capable of handeling. To give you an idea my 2 turtles are currently in a 100g tank running a Filstar XP3, which is actually undersized for them. As soon I as I find a job they are both going into a 250g+ tank with a DIY wet/dry. It's really sad that 90% of the stuff marketed in pet stores towards aquatic turtles is in no way sufficient and sometimes completely inappropriate.
For a small planted aquarium it's prefectly fine though. I believe brianclaw is actually using one on an 8g tank.