I have a few, some I got as a kid by mailing away for them.
There's a lot of ones now where the cachet (The art on usually the left side for those who don't collect stamps) is added on to an old cover much later. Some are pretty cool, some less cool. I'm not really into the added on ones, but most first day covers have become really cheap. A dealer at a show I go to had several thousand of them picture like 10 monster boxes- 5 for a dollar- It was almost the end of the show and he said I could have all of them for $300. Glad I didn't buy them.
I like the sports related ones, and the event covers that have been done for world series and other sporting events. Most of those are a bit better.
A couple years ago I picked up some signed ones from a collection a dealer had bought from a Padres executive. All star game cancels, and some with nice signatures. Some were on odd things like the ticket envelope from the box office and an all star breakfast brunch menu card. Not all signed, but still really interesting.
This is the guy.
http://www.legacy.com/obituaries/san...?pid=156487761