I am also among those that believe that The Wire is one of the best handful of shows on television...period.
I watched all of Season 1 when it was on, only a bit of season 2 (I thought it sucked because it dealt less with the street crime), a good portion of season 3, and a good portion of 4.
I recently started buying the DVD's, and I now love the show even more. I bought season one (still great, probably my favorite), Season 2 (SO MUCH better than I expected; should have watched it when it was actually on), and Season 3 (amazing). I just need to pick up the 4th season to completely catch up for the new shows. The Wire is one of those rare shows were I have no problem watching 4-5 straight episodes on DVD (if I have the time), and I won't get bored. OZ was also like that for me.