I usually only observe the eight precepts when I am staying at a Buddhist temple for an extended period of time, such as for meditation retreats, etc. Other than that, I find the five precepts to be sufficient. The main reason is that, to me, the extra precepts are simply there to foster a contemplative atmosphere and are designed to be supportive to meditation practice, but they tend to get in the way of my day-to-day life. For example, working a physically demanding full time job with a short lunch break makes the precept to refrain from "eating at the forbidden time" rather impractical. Besides that, as a lay-follower, I also enjoy things like listening to music, reading just for pleasure, watching movies, etc. Plus, I think that Ben makes a good point about it being better to observe the five precepts really well rather than observing the eight poorly.
"Sabbe dhamma nalam abhinivesaya" ().
(non-Buddhist related blog)