Front suspension or full suspension mountain bike I'm considering buying a mountain bike and have narrowed it down to two models (make and model aren't important for this question).  One model is a full suspension bike (front and back), the other is front suspension only.  

What are the advantages/disadvantages to going with a full suspension vs front suspension? (I'd prefer answers from people with experience riding both types of bikes.)

EDIT

To answer some of the questions looking for more detail...  The 2 bikes are roughly in the same price range, around $1000 USD (the full suspension is slightly more money), and my plans are to ride it primarily in some single track trails that are in the area.  There will be a lot of hills and rocks and such.  I doubt there will be any on-road riding involved.
If we assume that both models are at about the same price point, here is what you can expect:


The hardtail will be substantially lighter than a comparably priced full-suspension bike.
The hardtail will likely have a higher level of components and possibly a better front shock because of the increased expense associated with the full suspension frame.
Unless you can lock out the rear suspension, you will have higher pedal efficiency on the hardtail.


Your style of riding may dictate that an full-suspension is appropriate, so the best thing to do would be to describe how you want to ride to the professionals at your local bike shop and listen to their advice.

I have an admitted bias toward hardtails (and even fully rigid bikes). I've been riding a long time (since before mountain bikes existed...), and don't really ride anything seriously aggressive off-road (but I have ridden a lot of Utah Red Rock Desert).  I've tried several full-suspension bikes, but for my type and style of riding I've not been convinced that the added cost to get to a similar weight/component mix was worth it.