A few thoughts: One of the more stripped down sci/fi movies of the 1960s, Seconds is a John Frankenheimer thriller with some fascinating (albeit occasionally sloppy) cinematography, strong performances, and a cool concept that could have easily been made a lot sillier. Instead, they let it be pretty dark, and with the tight camera angles and paranoid tone, they manage to make it all pretty suspenseful and unnerving. I've never been a fan of Rock Hudson, but he did a solid job here, and I felt like I was watching the character, not just the face -- which could have easily been the opposite here. There are a lot of twists and turns to the plot, so much so that I don't even want to talk about what happens in the movie, but suffice it to say it's not standard '60s sci/fi by any means. In fact, I'd barely even call it sci/fi at all, as the way it handles the more fantastical elements is calculated and believable. Very dark, very suspenseful, with an ending that will stick with you.
Who would I recommend it to? This is the kind of movie I could recommend to pretty much anyone who likes older films. It's very Twilight Zone-esque, so most people will likely enjoy it.