I would say they are definitely a "must have" pattern. There have been many days when they saved my day. Like any pattern, there are also days when they don't work real well. Depends on the conditions.
As far as the question about the yarn matting together, this is not a problem. When they are in the water I guess it kind of billows out. The important thing is that it looks like the pupal shuck/casing of an emerging caddis. I have "sparkle yarn" that I got from a distributor that is closely aligned with LaFontaine (thebookmailer.com), and it is softer than the antron you normally get from fly shops. It doesn't hold it's bubble shape when it's wet (out of the water), but it works great.
Here's some tying tips from Paul Stimpson, who has been in videos with LaFontaine:
"The Emergent Sparkle Pupa is Gary's most popular fly pattern, however, I have found it is also the fly most often tied incorrectly. I have listed what I think are the most common mistakes and how to correct them:
Over-body is too thick. The body of Antron yarn must be sparse on this fly. You have to be able to see the under body through the over body. Gary's pattern calls for one ply of antron for the top and one ply for the bottom. On a size 12 hook, one ply is too much, I always pull a few fibers out of the ply for a #12 and a few more for a #14. When I tie a #16 or #18 I'll use half a ply.
Under-body must use a fur and antron mixture. Most of the ESPs I've seen lately have just Antron touch-dubbed for the under body. Because Antron is translucent (light passes through), If a trout is looking up it may mistake your fly for an empty nymphal shuck. Even a young trout will soon find out there are not enough calories in a shuck to make it a worthwhile bite. With a blend of fur and antron touch-dubbed, the body becomes more opaque and most closely resembles the newly hatched caddis.
Deer or elk hair wing is too sparse. Remember, this fly floats in the surface film. If the wing is too sparse, the weight of the hook and Antron body will pull it under the surface. The fly needs to be tied with enough hair to float. I put fly floatant on the wing but not on the Antron body. This enables the wing to be visible and the body to sinkjust under the film.
One more thought on the ESP. A fishing tip I got from Gary years ago. On a good caddis stream or lake, when nothing is going on on the surface, use the ESP as a searching pattern. Trout are used to seeing caddis all summer long and the reflective quality of this fly makes it a great attractor."
If you are interested in learning a whole lot more about caddis and how to use them to catch trout, I would highly recommend getting a copy of "Caddisflies" by Gary LaFontaine.