Great for Half-Life fans, most other people shouldn't bother.
The first thing I noticed is I could have a widescreen resolution as high as I want, which was expected, but its great to be able to play in widescreen now instead of having to play the game in squashed mode on my widescreen monitor.
When starting up the game and riding on the tram, the graphical changes arent very noticeable at all. It was just like playing the original game, in a higher resolution. The changes soon become apparent as you begin walking around that it uses the source engines lighting effects. Shadows are cast over characters faces and your weapons, and instead of shadows underneith models being blobs, there are now realistically cast shadows on the environment around you.
When you begin shooting, you'll notice ragdoll physics on your enemies. This was the most important change, for me, as i'm accustomed to ragdoll physics now, and older games often leave me unsatisfied when enemies die and just clip through the wall or something in an animation. Its very satisfying shooting stuff now, especially on the canyon levels when you kill soldiers and watch them fall over the edge.
Other updates include nice water effects, normal maps, and other stuff mentioned on wikipedia that I either don't know what they are or didn't notice them.
With the exception of the physics, none of the other graphical updates are very noticeable unless you really stop and look for them. And overall it hasn't recieved enough of an overhaul to really be recommendable to people who have already played Half-Life and arent interested in playing it again.
It also has a few problems too. Right after everything goes pear-shaped in the facility, you are able to smash a computer device on the wall to open the big doors leading back to the tram, and there is a scientist holding onto a big pole and eventually falls to his death. In the original game I always went to watch this bit before continuing, and this playthrough was no exception. But there seems to be a bug where sometimes the doors don't open back again and you're stuck there with nowhere to go except to load in your previous save. I noticed other things too such as parts of walls missing letting me see into the game void, and sometimes the ragdoll physics would mess up leaving a baddie sort of vibrating on the spot after i'd killed it. Some sounds also don't seem to play over others. If I fire a weapon while an alien is screeching sometimes I dont hear the weapon fire. These can all be forgiven, but in retrospect Valve haven't done a whole lot with this remake, so they could have at least made sure everything works how its supposed to.
If you've never played Half-Life before, then definately buy this version. You'll get to experience this amazing game with some nice little modern touches, and for just $9.99 on Steam its completely worth it. For people who already own Half-Life, it really isn't worth buying it again because the changes arent significant enough for another purchase. For die-hard Half-Life fans like myself, of course its worth buying and playing this, but you dont need convincing because you probably already did!
I still would have loved a complete overhaul like Counter Strike: Source was... I still hate that game and its a crime that it was fully remade and Half-Life wasn't. But now i'm looking forward to the Black Mesa: Source modification for HL2. Hopefully that will be really fun to play.
Info here if you don't already know about it:
http://blackmesasource.com/