In my case, it's 100% accurate for tracking my SNG results over PS and FTP, which covers hundreds of SNGs on both sites. I does miss a lot of the micros that I played on Everest early on.
I'm pretty sure it's extremely accurate in most cases where it matters. I'm positive that it becomes a really valuable tool for table selection when you play $22 SNGs and higher. Below that, there's no need to table select and you can consider all villain as fishes and be pretty accurate.
One thing that can be misleading is the fact that they give two different numbers for ROI. "Avg ROI" averages your ROI over all games played regardless of the stakes. "Total ROI" takes stakes into account.
So let's assume you play 2 HU SNG. First one is $1 stake and you loose for -100% ROI. Second one is $1000 stake which you win for +100% ROI (not taking rate into account). Then you "Avg ROI" would be 0%, while you "Total ROI" would be 99.8% ($2000-$1001=$999 return for $1001 investment)