I actually used Direct Energy for a while, and right now am on their competitor Xoom energy.

When you sign up, you're locked into a rate for 1,2,3 years. This only affects the supply rate, so if eversource raises T+D charges, you'll still be hit with those as they're listed separately on the bill and charged by Eversource, not Direct Energy. If Eversource lowers the supply rates and you want to switch back to them, you'll sometimes have to pay an early termination fee. They're typically not crazy, I think the plan from Xoom I'm on right now is like $50, but something to keep in mind if you're signing up for a longer term. However, when Eversource raises the supply rates, you'll still be locked into your lower rate.

Another thing to note is if you plan on getting solar panels installed, or an electric hot water heater installed (one that is billed separately), you might have issues with one of these plans depending on how the terms are written, so read carefully or give them a call before signing up for anything.

TLDR; it's great so long as you read the terms and conditions and know what you're signing up for.