A very common question. It depends what benefits you think you will use.
Benefits for Gold are:
1) Free checked bag (also available with some credit cards)
2) Earlier boarding (if that matters to you)
3) Free same-day standby
4) Free upgrades on flights under 500 miles, but you'll be at the bottom of the list (although it's been known to happen)
5) Ability to earn 500-mile upgrades, or buy them, and use them on longer flights - but again, you'll be at the bottom of the list
6) Access to preferred seats, and Main Cabin Extra 24 hours before the flight.
A couple of years ago I bought Gold, because I'm often at the airport early and would like an earlier flight, but mainly because on the particular flights I take there's a particular seat I like to book that's designated as MCE.
My personal opinion would be that if you fly a lot of short flights, especially connections, it can be worth it (you'll occasionally get upgraded but more importantly you'll be able to switch to an earlier flight and avoid delays when they happen on later flights). Otherwise no. Remember that most of the benefits can be purchased ala carte.