I sorta lucked into my main run of 1-544 for Uncanny X-Men, with buying someone's run of 2-150 + GSX1 to complement my sporadic collection of 200 to 300. Picking up that run spurred me into wanting the complete series which I managed to do over 2-3 years, with me lucking into an inexpensive copy of #1 early on. Did the X-Men (1991 series) at the same time, mostly due to my love of Jim Lee's art which pulled me into serious comic buying.
So I wanted the challenge of picking up the variants (Canadian price, UK price, direct vs newsstand, etc) and got very deep into that as well, though I started getting stuck on a couple of the $0.30 and $0.35 variants. What's funny is the massive influx of cover variants late in the run (500 onward) started ticking me off and it spoiled my appetite for completing all the variants and having a perfectly complete set, and I stopped at that point. Off the top of my head I'd say I have ~75% of the variants.
I really enjoyed the challenge of collecting the initial run, though not sure if I'd do it again. Actually debating selling off the run, as I change up my collecting goals now. We'll see.