If you're buying pre-construction I wouldn't use an agent. The agent will promise things like getting the builder to agree to add a couple of thousand dollars in upgrades, amongst other things, but you'll likely never get a reduction in the purchase price. Would you rather have 10k off the purchase price without an agent, or have 2k of free upgrades with an agent? The numbers are just an example, but the builder has more flexibility to bargain with the purchase price knowing that they won't have to pay your agent a cut.
The bottom line, in my opinion, for re-sale you should absolutely use a real estate agent, you'd be silly not to, but for pre-construction the pros far outweight the cons to not use an agent and purchase it by yourself. Of course an exception is if your agent can get you into a VIP viewing.