We have done this in the past here are some of the pros and cons:

Pros:
-frees you up for core activities
-warehouse invests in new technology & other tools that you would not normally invest in which makes them more efficient

Cons:
-Can be pretty expensive
-Quality becomes a factor you have to make sure that they are not serving multiple clients with similar products
-Risk of them using the mailing info to sell your clients other products