As Maclolm says, the 515 has a narrower spool than the 525. I bought one about two weeks ago, and honestly can't fault it. However I only use it for clean beaches. It casts very well and smoothly. Retrieve is fast, but I wouldnt use it over heavy ground myself.
Having said that, if your fishing into kelp beds and don't need a long cast, then you could load it up with heavy line. They are built like trucks and would probabaly withstand some heavy work.
Personally though, I have mine loaded with 15lbs mono and 60lbs shock leader. When used with a suitable light beachcaster or bass rod they are a dream.