It is well worth picking up Ben Gunn, As far as I know, there is no demo for this game, but you can get it for fairly cheap now. I don't pay much attention to a lot of the reviews, most of the review sites out there won't give a game a good review unless the developer pays for it. If they don't, then most of those reviewers give it a 10 minute trial and write up something based on what they experienced. The game is not perfect, but the fine modders of Seward did an excellent job with this one mate! They packed a whooping lot of content into this game! I have been playing it for over a year myself, and still haven't seen everything they packed in here!
The crashes are a bit overstated, the game does crash occasionally, but not near as much as some have said. As long as you install outside of the default Program Files directory, and turn off some badly written codecs, the game should be fairly stable. There are some bugs, but most of them are not game breaking, and our mods take care of a lot of them. Luke & Co. have done a fantastic job with the mods!
If you have played POTC, the gameplay is very similar, it is based on a very much improved version of the engine. There are some major differences however that make this game quite a bit more challenging, the biggest of which is the P.I.R.A.T.S. system that is used to set your skills. I won't go into great detail mate, but if you try and sail a ship that is above your current navigation skill level, your other skills will take major hit, even when you are on land. I think this is where most people say the game is to hard, well, when you have a -40 to all your other skills because you are trying to sail a frigate when you only have the navigation skills to sail a sloop, that does make the game almost impossible to play.
If you do decide to give it a go mate, I do recommend what Luke said, play a bit first without the mods till you get a feel for the game. There is an endless supply of things to do in this game! If you do find yourself getting a bit bored, chasing down the various gold fleets is almost as much fun as hunting down the Spanish Treasure Fleet in Sea Dogs!