Slow73: I have found lead putty to be quite useful in both stream and lake fishing. I use it often, but not exclusively, when I need to add extra weight to my leader/tippet.
Advantages: 1. It goes on and comes off the line easily. I can quickly switch from chironomid fishing to dry fly fishing without having to deal with split shot crimped to leader by snipping line, sliding the split shot off, then retying.
2. You can easily customize the amount of weight you want on the line. If you are nymphing a stream, your success will obviously be related to your ability to get the fly near the bottom where the fish are in sheltering lies in the swift current. You can do this to some extent by lengthening or shortening the distance from your strike indicator (if you use one) to the fly. Often, it is more effective, efficient and fast to just add a little more lead putty to the tippet knot. I keep a dob of putty on the butt of my rod, and move putty to or from the tippet to the butt as needed. If I'm not deep enough to get hits or occasionally bump the bottom, I add more.
The real advantage of putty shows up when you have too much weight on. If you hang up on a rock, you often will just pull the putty off of the tippet knot and still have an intact rig, whereas with split shot you are more likely to break off everything distal to the split shot. If you have just nymphed a deep, fast run and move to shallower or slower water, it is easy to just take a little putty off. It is more of a pain to get rid of extra split shot on your line, even the kind with the little wings that you can squeeze with your pliers to supposedly open the mouth again. Even when using removeable split shot, I can get the putty off and be fishing again in less time than I can fumble around in my vest and get the pliers out.
3. Fewer break-offs (in my hands, at least). Crimping split shot on thin leader or tippet can weaken the monofilament at that point, especially if you are a bit heavy-handed when you crimp it on. If you don't crimp it on firmly enough, the split shot can slide up or down or rarely even come off. Putting putty on the tippet knot doesn't compromise the integrity of your knots or line.
4. Faster rigging and unrigging = more time fishing. This advantage is really a subset of advantage 1 listed above. Not only is there less hassle rigging/unrigging, but set-up/takedown time is reduced.
Disadvantages: 1. The greatest disadvantage is advantage 1 - It comes off the line easily. When you first start using putty, you might find that you fling it off the line from time to time when you cast. Like everything else, experience will help you avoid this problem. A more gentle, open loop cast helps alot. The stiffness and adherence of the putty correlates with its temperature. Colder temperatures harden it onto the line more. I only rarely lose putty from the line now, and know when to use split shot instead.
2. Variability in quality from product. I have tried several products, most of which were the environmentally friendly lead-free products and have not been happy with most of them. Loon makes one that is ok, but I have settled in on Shape-a-Wate, made by Ligas. It contains lead. If you are uncomfortable with that, I would use the Loon product.
Old Man, while you don't NEED to put extra weight on, I have found that under certain circumstances it improves my catch rate and the size of my catch. If you are fishing over relatively uneducated fish or more agressive feeding subspecies such as cutthroat it probably doesn't make much difference. I agree. Avoid the hassle of more difficult casting and potential tangles by just using a weighted fly. There are two circumstances where I use weight on the line above the fly:
1. If I am nymphing a stream with educated, picky fish in it. Think about drag-free drift for a moment. What is going to have the deepwater version of a drag-free drift, the relatively straightline motion of a heavily weighted fly, weighted heavily enough to get deep, or an almost weightless fly on 5X or 6X tippet 18-24'' below a weight on the line to get it deep enough. The wnweighted fly can move naturally with every microcurrent generated by rocks on the bottom. This without question helps me catch more fish and bigger fish in certain streams and rivers...........and would make absolutely no difference in other streams.
2. Chironomid fishing in heavier wind on lakes. I love to fish chironomids when the circumstances are right and have been quite successful at it. I usually fish Tungsten beadhead chironomids under a strike indicator if I am fishing near the bottom. Several times this year, I would be catching fish after fish in a mild wind, then my catch rate would slow down when the wind picked up. As more wave action was generated by sustained higher winds, current action would lift the fly, which would no longer be straight down. Under these circumstances, I add some lead putty to the tippet knot to get the fly back down through the current near the bottom where it belongs. On occasion, I have to go to a bigger or more bouyant strike indicator to keep the fly bouncing enough to entice the fish and to keep the indicator from sinking from the combination of increased weight and wave action. when I have done this, my catch rate has gone back up because my fly is "in the zone" again. TRY IT! YOU'LL LIKE IT!