In answer to the Gareth: NO - you dont have to have any other games installed, this is a complete stand-alone set of games! Go get!
This really is a snapshot of gaming you cannot miss. I missed out on HL2 when it first came out, and after playing through the first few sections of it last night I must say I am so impressed, it is still cutting edge even after this time. The pacing and sense of chase that you feel is truly amazing. Its just a rollercoaster from one piece to the next. It feels like what it must have been like from Keanu's POV in The Matrix!
Although its a linear game, it absolutely never feels like it - achieved through clearly the best ever level design - it really is so noticable it makes me wonder why more games havent got is AS right.
This is going to be a great journey - what with 2 more episodes after this (+ you get a free extra level offered to D/L through Steam too! "Lost Coast" - which is like a playable tech demo).
Portal will tax you at first but you certainly start to look at things differently after messing around with that game!! Genius!
And the icing on the cake with TF2 - a great blast of MP fun... not very subtle in terms of gameplay, but probably the most 'polished' looking game I have ever seen. The visual style and audio in the game are just spot on for this games character and at last proof that it is possible to launch a PC game with ZERO bugs! (Oh, actually there is 1, the lobby list is very unresponsive and its just random kuck if you get into a server or now by repeatedly double-clicking any of them!)
This is my first review - I just had to reply to Gareth so as this point didn't put people off (of reequireing HL1). But when gaming is this good (and cheap) who needs DX30 Shader level 139 etc!!