I have had a POD HD500/x for almost 5 years now. It is paired with a DT50 and the sounds are very convincing. I have also run this direct to an amplifier with good results as well.
First off, there are people out there knocking the product because they can't get it to sound good. I have been a Line6 user since early 2003. It started off as a recording experiment and eventually found it's way into the live setting. I feel like I understand all the parameters of the units in detail, so I can expect better success than someone new to the product line. I have owned all of the PODs in succession with each of their releases (2.0, XTLive, X3Live, HD500, and HD500x). So if you decide to purchase let me know if you have any issues and I can point you in the right direction.
Secondly, I play in more of a "variety" type band that requires several different types of options (effects or all out different aspects in tonal characteristics). If you don't require much, or only going to use it for a couple tones, there are other options out there that might be better in the long run. Maybe the Line 6 M9 or M13 for effects combined with your one good to go tone from an amp. Just an idea.
I do feel the tones from the HD500 or HD500x do have rather authentic tones. I am a bit of a tone snob, and do own a 65amps Empire head. I do like good Marshall Plexi tones. While my ear my not be as good as some, if you are an average gigging musician, most people in bars or county fair events can't even tell a difference. It does perform really well in live settings as long as you understand where your frequencies lie in a full band context.
I know this is quite long, but I didn't have this information going in on my purchase. I hope this helps you make an informed decision. If you have any other questions, don't hesitate and let me know.