The hellgate beta test ends on Friday Oct 26th and the game is on track to go live on the 31st. Prior to launch the beta will reopen so that people who aren't able to get their copy on day 0 will be able to play until they do. Characters made during this second beta will be carried over to the live version.
The game is a true spiritual successor to the diablo series although the storyline is not nearly as strong or predominant. The game features a decent number of missions and pseudo random areas to keep players busy. At max level, whatever that turns out to be, you can still anticipate spending many hours perfecting your gear if that's what you're into. The game is fun, and easy to get into. It should be a fairly successful. I wouldn't want to try to predict how many people will want to go all in with a monthly subscription fee though.
A full review and details on what your subscription dollars entitles you to can be found here.