I've got SPDs (Shimano 520 or 540).

Positives
I like that they're double sided so I don't have to find the clip in side.
I like that the shoes allow me to walk around pretty comfortably. When I commuted, sometimes I'd forgot I had 'em on until hours later!
They're cheaper than full on road pedals.
I like their durability. Their built to take the abuse of mtn bike riding! Note: I've not had road pedals so this is just my perception.


Negatives
A bit heavier
You don't have as much surface area and thus might find it less comfortable thought I've never had a problem.
Less surface area also means not as efficient in power transfer likely.