I run primarily on trails in my neighborhood in Southern California. In June, my son and I are running the Dipsea Race. I have never owned a pair of trail running shoes, but they seem to be getting increasingly popular. I'm not really sure what the advantages/disadvantages of them are, particularly on a course like the Dipsea, which apparently is fairly technical and rugged. If the advantages are minimal, I'm not inclined to shell out the money for them. Any advice would be much appreciated.
I'm also interested I'll give u a bump
With trail running shoes you will get more protection from roots and rocks (in the soles and at the toe), better traction for uneven/wet/muddy/snowy surfaces, and uppers designed to keep sand and other debris out. Naturally, they are heavier, although minimalist models are becoming popular. My suggestion is to try a pair on terrain that mimics the Dipsea and see if you prefer them to your road shoes.
LC summarized it well above. 2 more points: 1) lower likelihood of rolling an ankle; 2) Gore-Tex models will keep your feet drier when running through water (at a trade-off of being sweatier on hot days).
For a race, deciding factor for me largely hinges on likelihood of encountering long stretches of rocks, water, or mud, whereupon I'll take the weight penalty.