In MA it does not affect net metering, you still get your summer credits based off your utility's rates at that time. But when you buy back your energy, you get it at the third party rate (hopefully lower). Note these suppliers sometimes have lower starting rates and raise them after your contract. Sometimes this isn't the case, but if it is that's why it's good to be hunting around for cheaper rates before your contract is up and why it's nice to not have a termination fee.