Am surprised by lumpster's post. Must have beeen a weird toddler group - Lumpster, suggest you try Kangaroo club at the URC, very welcoming (Anita who runs it is lovely.)
We've been here, ooh, must be coming up to seven years. In my experience, Harpenden is great, green and pleasant environment, friendly people, plenty to do with kids but none of the stress of London (and half an hour on the train to town when you want a City fix). We've got lovely neighbours and feel part of a real community, especially through ds's school. We can't walk down the high street without bumping into a handful of friends which is lovely for ds.
Only downside is that most people are quite well off so if you aren't, I'm sure it would grate a little. And stuff is expensive - as soon as you give a Harpenden postcode to any company I'm sure they whack up their charges (builders, AA driving school etc. etc.)! Oh, and it's full of Tories given high house prices and % of City workers (Peter Lilley is the MP). That may not bother you, of course...
I don't personally find that people are snobby but I'm sure there are some people who are. I probably just don't move in their circles, being a mere pleb.