When I reviewed That Touch of Mink, I mentioned that 60s comedies were either a hit or miss for me- some I've absolutely loved and some I'll never watch again (That Touch of Mink being part of the latter). I wanted to see How to Steal a Million because I'm trying to see more Peter O'Toole films and also because many of the people I follow on Tumblr absolutely love it. After watching it myself, I now understand all the hype.
This was the only time O'Toole ever co-starred with Audrey Hepburn, which is quite unfortunate because I thought they were wonderful together. While their performances are great, I felt that the best aspect of the film was their chemistry. I've heard some people criticize the film for its "fluff", but isn't that what makes most romantic comedies so much fun?
Overall, if you're looking for a film that's lighthearted, romantic and fun, How to Steal a Million is just that.