All depends on the one you buy. Also check out any threads on various forums, that deal with problems, on a particular mod. Also check on customer service. Some mod makers are a make and forget. Other stand by their product.
BCV makes a good mod and has excellent customer service.
VapourArt makes a good mod and has good customer service, especially through there distributor Domo.
Mcr - ditto
Just to name a few.