Yeah...it looks like there's six episodes, and I'm already on episode 5. Last night's cave antics were on episode 2, and I've only played for about two hours since then, so you can blaze through it pretty fast. =/
It'd be good as a rental, but I wouldn't really recommend it to buy.
The graphics are good, but some of the voices aren't even decent soundalikes (I think some of the actors actually did voiceovers, but Charlie and Locke sound nothing like them). Exploring is decent enough, especially for coming across stuff like the numbers and random Desmond/Dharma stuff inside the hatch (not really spoilers...only mentions of early season 2 stuff, but I blocked them anyway).
Worth checking out if you're a fan...just probably not to spend £40 on.