Really funny that I just saw this post. As I had done the exact same thing, and have just completed Angel of Darkness tonight! Started about 2-3 months ago. All my life, Tomb Raider 1 had been the only tomb raider I had ever completed, yet I bought the whole series.
I played through the Gold addons as well, except for The Lost Artifact, TR3's Gold pack. I've just bought it off Ebay and hopefully I'll get to play it next week.
Here's a few notes about my Tomb Raider marathon:
*I played Tomb Raider 1 on the PlayStation, despite owning the PC version. This was before I had Glidos and so it was my only option. Was nice hearing all the music though! In fact, it was buying this second hand playstation copy that caused me to start my marathon.
*I admit that for both Tomb Raider 2 Gold and Tomb Raider 3, I downloaded gamesaves to play through the bonus levels because hell if I was gonna go hunting every secret!
*As you guessed from above, getting all secrets wasn't my priority though I picked up what I found.
*Haven't played TR3 Gold yet, but its been ordered!
*Tomb Raider: The Last Revelation was surprisingly REALLY good in my opinion. Extreme Tomb Raider stuff with an epic feel to it. Great atmosphere. This was the one that probably took up most of my time as it was the most complicated. Well worth it though!
*Chronicles was EXTREMELY short in comparison to all the others. Especially when playing it right after Last Revelation. Though admittedly, after such a long hard time in LR, this one was like an enjoyable teabreak.
*Angel of Darkness, I really enjoyed. Very movie-like and good atmosphere, just a pity about the controls. Felt a bit rushed in the last few levels, but it had a nice long boss fight with good music that was a nice ending to the whole marathon.
Hope you enjoy it as much as I did, Zardoz!