The game is awesome enough to play through again.
Agreed. I played through it 4 times simply because it's so friggin' entertaining and darkly humorous.
Do you have to reinstall it as I watched the ending and it was the same?
If you have it installed through Steam (which is the only way you can have it installed IIRC), it should update automatically unless you changed options to turn off the game's automatic update. If you right-click the game in your list and choose "View Update News" you should see the following:
[-]March 3, 2010 - Portal Update Released
Updates to Portal have been released. The updates will be applied automatically when your Steam client is restarted.
Portal
Added valuable asset retrieval
[-]March 1, 2010 - Portal Update Released
Updates to Portal have been released. The updates will be applied automatically when your Steam client is restarted.
Portal
Changed radio transmission frequency to comply with federal and state spectrum management regulations