The base mould isn't very hard to transform at all - if you like complex transformations you'll be disappointed, but the upside is that both modes are solid. It's very intricately sculpted, so it's an interesting piece to look at.
In terms of screen-accuracy, it's actually based on early concept art that Josh Nizzi did for RotF - HasTak must have picked it up in order to have a Megatron on shelves when the movie was released. The final CGI model had different arms, but it's otherwise fairly accurate.
Articulation is somewhat limited. Since it's all one big cannon, the right arm only has shoulder articulation - forwards/backwards and in/out. The left arm has a good range of motion in the shoulder, and the elbow and one large finger move. The torso is one solid block, due to the electronics and MechAlive gimmick, and the head hardly moves side to side (partly due to the shoulders and partly due to said gimmick). Fortunately, the legs have a fair amount of motion in them, and the 'feet' are big enough that you can stably put Megatron in deep poses.
Given the prices this mould is going for - and has sold for - on eBay, $185 for a brand new one with an interesting new paint job and a limited release actually seems almost reasonable.