It always depends on what you are trying to download. If you are trying to download some piano sonata that is almost unique on the network your chances to complete the download are far lower than if you were trying to download some Robbie Williams song.

Personally, I find LimeWire quite reliable and almost on par with Fasttrack. I can't tell why you are so unlucky, my downloads tend to complete at least in 50% of all cases once the download started.