Surveys and focus groups are good tools to evaluate users' interests and feelings about a product (or potential product). They can be a starting point for determining areas you want to focus more in depth research like observations and contextual inquiry.

Surveys

Strengths

Getting general feelings about an existing product
Acquiring demographic data on target market
Understanding what users "think" is important (marketing data)

Weaknesses

Survey design is very difficult to get right
Constrained responses
Typically limited time & focus for response
Limited or not ability to follow-up for clarification

Relies on user to self-report accurately

Survey Design Resources

Focus Groups

Strengths

Can talk to many people at once
Allows users to feed on each others' ideas
Can help understand users' attitudes, beliefs and desires
Useful for getting users' reactions to ideas or to prototypes

Weaknesses

Not great for understanding what people REALLY do with a product
Not good for understanding what features people will really use in a new product (only what interests them)
"Group think" drives people toward consensus with the loudest opinion
Relies on user to self-report accurately

Focus Group Resources

The Use and Misuse of Focus Groups by Jakob Nielsen