Literally no reason not to play it if you like Portal. The number one reason to get this game is that, if you have Portal 2, it's free. I really don't like using walkthroughs but spending an hour on a puzzle isn't super fun either. or something, because the puzzles are REALLY hard. One thing I wish this game had was at least some kind of hints. This could be a bug with Source, but I haven't had the alt + tab'ing issue with other Source games.
One problem I've had with this game is constant unexplained crashes, especially when I alt + tab or change settings. The writing isn't super on-par but everything else is exactly portal - like, if it weren't for the writing and the voice acting (no offense to the writers and VA's, it's hard to live up to Valve standards & budget) it would and could definitely be a new portal game.