I love these films. And because they were produced by the Bond team their production values etc. are fantastic. Ipcress is still probably the best spy film ever made. (and features one of John Barry's best scores.) FIB is a good 60's spy film that isn't campy. Billion Dollar Brain is Palmer on acid. (it has some surprising staying power though.)

Haven't seen the 70's film, but the two from the 90's are interesting curios. They aren't that good however. There are versions floating around online and they were recently released together in region 2. Worth it for serious Palmer fans only.

Michael Caine is the man. No one can touch Harry Palmer or Jack Carter.