I've just finished Portal 2 and I highly recommend it.
The basic idea is the same as the first, use dual portals to solve puzzles. I found the plot of the sequel more compelling and fun overall, and I think the puzzles were more varied and difficult. The end bit was easier but that's fine with me since I found the first one rather difficult.
Overall I easily recommend anyone buy Portal 2, especially now that it's half-price (compared to initial release). It's a relatively short game, but it does have a separate co-op mode, Valve is releasing free DLC in mid-September, and it's more fun than many longer games I've played. It's also available for both Mac OS X and Windows.
Enjoy.