I'm currently playing through HL2 again using FakeFactory Cinematic mod v9.5 with HD addon. Seems worthwhile, the game looks good to me, although I can't really remember exactly what it looked like before. Loading times are pretty poor and judging by the memory usage I think people with only 3GB RAM may experience some stuttering.
Here's my take on the levels I've played through so far (a general appraisal of playing HL2 again with FF):
*****WARNING: POTENTIAL SPOILERS BELOW*****
Point Insertion / Red Letter Day 5/5
I've grouped these two sections together as they pretty much lead on from one another. The graphical quality is immediately apparent in City 17 and this section of the game is a tribute to what Valve do best, setting the scene with some nice scripted sequences such as when you escape across the rooftops
Route Kanal 2/5
Probably my least favourite HL2 chapter, your lack of firepower is somewhat frustrating and in general this level feels quite dated. The puzzles, such as the see-saw you need to counter balance with bricks seemed OK at the time but nowadays the level feels rather boring in general. The combine also seem more effective than I remember here.
Water Hazard 3/5
In general this level is pretty decent with a nice mix of frantic driving, physics puzzles and indoor combat. Didn't feel quite so long this time around.
Black Mesa East 4/5
Another nice interlude, building up the story and introducing you to the mavity gun in a friendly way outside with Dog. A lot of developers could learn from this in terms of how they present 'quirky' weapons to the player.
We Don't Go to Ravenholm 4/5
This is quite a turnaround for me, as in general I found Ravenholm rather frustrating first time out. Took me around half the time to finish this time out as I didn't get lost/stuck on the rooftops. Great atmosphere and a nice 'sandbox' level for the mavity gun. Nice section at the end of the level with enemy sniper rifles too, which are sadly underused in this game until you get to Follow Freeman.
Highway 17 3/5
Bit of a mixed bag this. The buggy feels a bit artificial in terms of how it drives, and there are times when it gets stuck unnecessarily. On the plus side, this level features some great set pieces, such as the crane section. The bridge section is quite atmospheric and almost scary at times, I was very close to giving this section a 4.
Sandtraps 4/5
The lighthouse section is another great set piece, and there is a nice change to the game mechanics here, making you think about where you are treading. It's got a nice boss fight (my type of boss - no excessive gimmicks, just loads of health and a chance to use up some of my stockpiled ammo!) together with a nice reward. Having antlions at your disposal gives you some extra tactical possibilities. The assault on the entrance to Nova Prospekt was arguably my favourite section from my first play through, but this time it doesn't seem quite as frantic. I've probably been spoilt somewhat by more modern offerings such as CoD4, making the scale of it not quite so impressive as it once seemed.
Nova Prospekt 4/5
Quality level, very good visual setting and some nice fights including a couple of antlion guards. The use of turrets helps freshen things up again.
Entanglement 2/5
This level doesn't really bring a great deal to the table, it's just an extension of NP really. The unique feature is use of friendly turrets, but those sections can be a frustrating level at times. I also experienced a bug (due to FF, almost certainly) where I had 2 Alyx's running around.
Anticitizen One 3/5
Nice to be back in City 17 again, this time with combat and rudimentary squad control. Sadly your allies have a habit of getting in the way, even if they do sometimes say sorry afterwards!

Overall the level plays out OK but just seems to lack that extra something, the final section as you climb a bunch of gantries and go over some radioactive slime is a bit tiresome too.