I mentioned on another thread that I sold my Spitting Image DVD's. I sort of regret buying them, as I ended up fast forwarding them a lot and the majority of stuff I just couldn't find funny.
Home Improvement was another one, I loved it back in 2006-7, but then they stopped releasing the DVD's in the UK. They suddenly released Series 4 after a long hiatus, I bought it, but only watched it once, as I then saw they weren't releasing anymore.
I used to only have the first series of London's Burning and the first three series of ER. I thought I'd regret buying those, but having gotten more into those shows, I love them! I now have the first five series of both shows, and if they continue to surprise and entertain me, I will continue to buy the box sets.