I'm sure it'll sell a ton, and I'll pick it up myself when Day of Defeat: Source is done, but I have to wonder how much more impressive this game would have been if it had arrived last year like they planned. A few games have stolen some of the thunder since then.
I think it's got plenty of thunder left. If you want to compare strictly on graphics, its main competitors were Far Cry and Doom 3. It's about on par with Far Cry, and even the fairly conservative art updating in Countersrike Source impresses me more than Doom 3 does. (It helps when the beautiful bumpmapped high-res texture high-poly scenery is NOT plunged into pitch blackness, you know?) If HL2 steps up from where CSS is, it'll be plenty impressive.
In the end, though, it's about delivering a fun and immersive game experience. If they can do that then they'll have succeeded, and initial reports say they've done that. It won't be the second coming of jesus but it should sell well and work nicely.