The Over-the-Hill Gang was an average movie for its time. Nowadays, however...it's hard to say, really.
The majority of the film's jokes are funny, the (back in 1969) A-list cast act pretty well, and the Western sets are as realistic as could be done back then. In short, the production value is good, though dated. It's the story, I'm afraid, that suffers.
The concept is intriguing but is played out...slowly and steadily. If this were a drama, it'd probably be okay, but it's a little hard to keep watching a snail-paced comedy. The ending is okay but is quite predictable; 30 minutes into the movie, you can probably predict the ending perfectly in your head.
All in all, The Over-the-Hill Gang is a so-so movie which will likely make you laugh a few times. I will warn you, however, that you might forget most of the plot within a week of seeing it.