Let's see...
Jim Belushi was great!  Meet him outside a restaurant in Chicago -- I was going in, he was coming out.  He said hello, asked how I was doing and chatted for a minute before getting into a waiting limo.  The sad part was I did not even recognize him until after he was gone.  But such a nice guy
Joe Manganiello (Alcide on True Blood) is super nice!  And much more attractive in real life

  Went to see a play he was doing in San Diego (pre-True Blood) and made plans via Facebook to meet up afterwards.  Gave me a hug as soon as he saw me and even introduced me to his family like I was an old friend.
Richard Thomas (John Boy Walton) is also a real sweetheart.
Doris Roberts was nice, but seemed a bit out of it 
All four member of Boyz 2 Men are nice.  Bryan Abrams from Color Me Badd was pretty nice too.  And Debbie Gibson was super sweet!  She spent some time talking to my niece when we were at an 'Nsync soundcheck (she was the opening act).
Oh, and I can't forget Ted Turner!  Met him at his restaurant in Atlanta -- he was really nice, but so drunk that I doubt he remembered talking to us 
Went to school with Rick Mirer (former Notre Dame quarterback -- played a bit in the NFL, I think for Seattle and Chicago) -- not sure that he would be considered a celebrity, but total jerk.  Has an ego the size of Jupiter.