When users have an issue they have a choice of whether to quietly leave your product or contact you and let you know of their bad experience. You want to make it as few clicks as possible to contact you and send you a message you can quickly respond to.

The best is a one-field form that is easily accessible - if you can avoid asking for anything but "Tell us what's up and we'll fix it for you ASAP" that is the best. Reassure them this message is sent to a real human's email account.

Uservoice, Kampyle, and other feedback plugins are tempting but misleading and they make users feel like their feedback is going into a black hole database. By sending you a direct email this ensures that it is in your inbox and hard to ignore.