I have a 15' X 3' island. It has 2 canned lights at the end, track lights AND 4 halogen lights in the island hood.
It's plenty of lighting!!!! But....the look of large hanging pendants is very decorative! Go for it!

I have a 12-foot island. The pendants I'm putting in are also about 10 inches in diameter and take 1 bulb. I think it will be plenty of light for task lighting on the island. I have plenty of recessed lighting for general lighting. My kitchen is already very bright without the pendants!

I have 9'3" x 42" island. 2 CFL cans at fridge end, 1 at open end (how'd that happen?), 2 more on work aisle side. That's my general lighting for kitchen. I also have a row of 5 cans in front of the range/sink/DW and one more over the sink. I have an 8ft track with 3 pendants hanging from it for direct/task lighting of the island. Each pendant has a 60W incandescent in it right now (we rarely use it, and it's on only for short times like when eating at the bar, though I have turned those 3 on instead of the CFLs during the day when I have enough sunlight but I'm working at the island).