if you're trying to get below 10 feet or so, you want a sinking line, not just a weighted fly or leader. And in my experience, I don't want a fast sinking line in stillwater, unless I'm trolling or casting and retrieving a fixed length of line in fairly deep water, like a 60' cast that I want to ride at a 15'-25' depth.
For getting to 8'-20' in stillwater or slow currents, I like intermediate (slow) sinking lines. They help keep the line tight and the retrieve manageable, and the action of the fly is more natural. Fast sinking lines tend to fall to the bottom and belly down off of the rod tip, unless you're using them to cope with strong current.
For targeting depths of 5'-10' in lakes, I like a sink-tip line. But I usually need to trim the tip so it doesn't belly. For example, a line with a 5'-8' sink tip and a 4' leader can often be used very effectively in water around 6', especially with a deer hair fly like a sculpin or dahlberg diiver. The sink tip gets the fly down to the right depth without line slack or klunky weight, and the natural tendency of hair flies to float keeps them riding just above the bottom structure. treating the flies with floatant often helps.
Sink tip leaders are supposed to convert floating lines to sink tip lines, of course. But you need to match them carefully with your line, because if they're too heavy they cast worse than split shot. And the ones I've used always hinge to some extent, which is not great.