I am a MTB SPD wearer, even on a road bike. I only use them because on longer rides they prevent me from getting pain in the achilles tendon. They don't make me any faster, and offer no better grip on the pedal than my old flats with pegs did.
The downside of SPD's is that if you are coming off, then there is nothing you're going to be able to do to stick a leg out to regain balance or to try and break the fall. You're going to injure your hip when you crash at any speed while clipped in. Just ask my mate who came off on Easter Monday and spent a week in hospital, and now has plates and screws holding his left hip together. He'll be lucky to ride a bike again at the age of 57, and certainly won't be the competitive, fast rider he previously was.
My personal advice on SPD's is, if you can use flats without suffering problems, stick with them. Unless you have aspirations to be a pro..