fpdb is a long way form either HEM or PT3, but it's the only significant free tracker and has come a long way in the last year or two. It used to be essentially unusable, and didn't have a HUD. At one point I considered joining their development team, but decided I just didn't have the time to commit to it. Especially if I actually wanted time to play poker.
If you can't afford to buy HEM, then go for it. But HEM is a pittance to a serious player and will more than pay for itself in a very short time. So the only real base that fpdb has is with people who don't have the BR to buy something, are so new to tracker software they just want to get their feet wet, need a non-Windows tracker, and/or are passionate about open-source software and would prefer to make do with a less-functional OSS solution instead of a commercial one.