Some numbers from official sources

According to numbers published by the University of Iowa College of Dentistry the success rate for implants is remarkable: 98 percent for lower implants and 91 percent for upper implants. This difference between upper and lower rate is due to the higher bone density of the lower jaw and has nothing to do with gravity.

The success rate for implants decreases in patients who suffer from chronic problems, such as tooth grinding and clenching or systemic diseases, such as uncontrolled diabetes. Also, individuals who smoke heavily or abuse alcohol may not be ideal candidates for the procedure.

A single tooth, several teeth or all the teeth can be replaced using dental implants. The key to successful implants is the amount of bone in the jaw. The upper back jawbone area is one of the most difficult areas to implant teeth in due to less bone structure and proximity to the sinus area.

What you can do

Practice exceptional oral hygiene. Implants, artificial teeth and gum tissue must be kept clean. Specially designed brushes, such as an interdental brush that slides between teeth, can help clean the nooks and crannies around teeth, gums and metal abutments.

Seeing your dentist regularly after surgery also improves your chances for success. You might have to schedule dental checkups more often than before to ensure the health and proper functioning of your implants.

Oh yes, and stop smoking. Dental implant success rates for smokers are lower.

For the scientifically inclined a few links to abstracts of scientific papers published by the American Academy of Periodontology, which basically show good results for immediate implants. They also show clearly better results for lower jaw (mandible) implants than for upper jaw (maxilla) implants:

http://www.joponline.org/doi/abs/10.1902/jop.2004.75.7.982
http://www.joponline.org/doi/abs/10.1902/jop.2000.71.6.923
http://www.joponline.org/doi/abs/10.1902/jop.2003.74.5.695