They are excellent for light, stealthy nymphing, e.g. low water or spooky fish. They are light and land with little splash. The tubes in the kit will not float a heavy nymph in deep fast water.

Pros:
very sensitive
light with very small splash
easy to move up and down leader
doesn't kink leader
convenient kit

Cons:
a bit expensive
needs 3 parts - tubes, needle, yarn
won't float heavy nymph in fast, deep water