My first time DM'ing was a 2e Planescape campaign back in 1997. I had only been playing D&D for 2 years before that and it was only in a Dark Sun campaign. I didn't know a thing about vanilla D&D or the planes. But Planescape was still calling me and I tried to make sense out of the planes. I studied the books until I had a mediocre amount of knowledge on what "the planes" were. The factions sounded cool, the new races sounded crazy, & the thought of being able to travel to such different environments really had my imagination pumping. Everyone kept pushing me to hurry & DM it so I tried before I was really ready. It was a train wreck; a couple players even decided not to play under me anymore because I was so inexperienced

But when 3.5 came out, I started DM'ing again and have gone through a few new groups of players in the same campaign. I finally have a good set of players and we're going strong.
So my 2e PS experience isn't that helpful for ya, but I'm not having much trouble running a 3.5 version of PS. There's plenty of material to find for conversions. I just recently revamped the way I use faction benefits and I'm now using affiliations to represent PS Factions. I found some great custom faction affiliations someone on Enworld created. I'm reworking some of them to fit in my campaign better, but the guy did a nice job and affiliations work great for factions.
I'm still running things pre-Faction Wars. I love the factions and I won't run Faction Wars until I've ran everything I wanted to and when I feel the campaign needs a huge change like that.
The parts I like the most about PS is all the exotic NPC's I get to run & the exotic places the players get to visit. I admit that a planar campaign isn't much different than a normal campaign as far as adventuring goes, but the fact that there are no limitations makes it different. I can work anything into the setting with ease and it'll completely make sense. The setting also provides much more opportunities for roleplaying.
Running the Eternal Boundary adventure is a must. I highly suggest that, especially since you're doing a purely Sigilian campaign. It's low level and really good. It still takes players from Sigil to another plane towards the end but it's nothing major...it'll still be purely urban. The players will get to interact with the Dustmen, Bleak Cabal, Xoasitects, & then whatever other factions you throw in yourself.
You could probably run Harbinger House next. I haven't played it but I've skimmed through it. It's supposed to be good and it sounds fun. It's set in Sigil but it involves a bit of time in 2 gatetown which would still make this a totally urban adventure.
Here's a lit of the adventures that takes place totally in Sigil:
Eternal Boundary
Harbinger House
Doors to the Unknown (At the Shattered Temple)
Well of Worlds (The Mazes)
Dead Gods (Into the Light) - it's a shorter adventure at the end of the book. It can be run on it's own without running the main Dead Gods plot.
I ran Fires of Dis which I think is a really good PS adventure that just doesn't get much attention from PSers. You go from Sigil to a gatetown, then a lower plane to another gatetown. I thought it was very well written and fun to DM. It really teaches players a lot about different fiends and what it's like interacting with them.
I almost ran Deva Spark but it just didn't seem solid. It had some fun scenarios but it was very campy and the adventure seemed empty.
Other PS adventures that I haven't ran but I hear are good are Great Modron March, Tales from the Infinite Staircase, & Faction Wars. Most of the other adventures I've run weren't PS adventures.