"Each age group participant shall be permitted to wear a wet suit without penalty in any event sanctioned by USA Triathlon up to and including a water temperature of 78 degrees F. When the water temperature is greater than 78 degrees F, but less than 84 degrees F, age group participants may wear a wetsuit at their own discretion."
From USA Triathlon Rule Book - Wetsuits

Triathletes whom I run into and coach always want to know how much money they need to spend to achieve their triathlon goals. While there are several pieces of equipment that can legitimately improve performance, a wetsuit is something that potentially yields very significant results.

In most cases a wetsuit can not only save you 5-10 seconds per 100 yards, but also require less energy to do so. If swimming is your least favorite triathlon leg, a wetsuit can help. Not only can a wetsuit make you more buoyant, therefore correcting body position, but it can also add a sense of security if the water makes you uncomfortable. Since a wetsuit helps you float, it make it very easy for you to stay "on top" of the water.

When contemplating a wetsuit, here are a few options to consider:

Buy or rent
Ideally you will want to have your own wetsuit that you are able to practice and race in whenever you need it. Some specialty stores, however, do offer rental programs that will allow you to try out a wetsuit in a race or workout. The benefit of this is that it may help you to find a wetsuit that fits you best and one you are most comfortable in.

Full Sleeve or Sleeveless
When deciding whether to go with a full sleeve or sleeveless wetsuit, first consider the fit. If having neoprene covering your entire body is not something you feel comfortable with, regardless of performance enhancements, a wetsuit with sleeves would not be your best choice. Another key factor is where you will be racing. Is the water warm? A sleeveless wetsuit will keep you cooler. If you are looking for the extra advantage, go with long sleeves for added buoyancy. Wetsuits have come a long way with fit. Choosing a suit with sleeves often allows the same range of movement that a sleeveless will offer. Some wetsuits are constructed for different body types which is why it's essential to try them on and make sure they fit you.

Transition Time Factor
One disadvantage with a wetsuit is while it does lower swim times, it increases transition times. For swims that are longer than a half mile I believe the wetsuit always helps more than hinders you. The right time to wear a wetsuit largely depends on you and your abilities. If swimming is your weakest event, there might be very few times, if ever, that your suit will not help you more then hurt you. Case and point: If you save 30 seconds in the swim because of the wetsuit, but then it takes 30 seconds longer in transition one (T1) to take it off, that 30 seconds in T1 is still time at "rest" which will help you in your final two legs.

Getting In and Out--Practice and Practice Again
There are some products, such as BodyGlide and Sportslick, that make getting in and out of your wetsuit much easier. Apply these to your neck, wrists and ankles to help the wetsuit slide off easier. I recommend practicing beforehand. Take it to the pool a few times, or to a couple open water swims, and practice the T1 process before race day.

A wetsuit can be a very cumbersome thing to put on, so allow plenty of time before the race starts. You definitely do not want to be rushed. Why? When you're stressed and rushing you will dramatically start to increase your body temperature while trying to work quickly to get your wetsuit on, and you don't want to overheat. Be prepared and calm, put it on early, and then allow the top portion to hang around your waist until your swim wave start draws near.

Ultimately, whether you're wearing a wetsuit or not, the best way to get faster in swimming is to, well, swim. But if you are looking for a smart investment on the dollars-spent to performance-gained ratio, a wetsuit is one of the first items you should consider.