This has already been discussed several times at length. I encourage you to do a search on furled leaders and read the previous threads.
The use of furled leaders vs. conventional mono leaders is much like the tedious indicator/no indicator discussions. One is not better than the other and their success is largely dependent on properly matching the gear and technique to the situation.
Opinions in favor of furled leaders often take on the tone of promoting them as the ultimate miracle leader. That is definitely not the case. They do some things extremely well and other things rather poorly.
When it comes to fishing dries, emergers, dry/dropper rigs, shallow wet flies, shallow nymphs, and most small stream applications, yes they are a great choice and they will serve you well. When it comes to punching a bushy dry fly into a tight spot, they are unparalleled. If you need to present a small dry on a long fine tippet, again they do very well. However, it is a matter of finding the appropriate combination of leader and tippet.
On the other hand if you need to do some deeper nymphing, tight-line nymphing, and streamer fishing, put the furled leaders away and run plain ol' mono leaders. They are just a better choice in general for this sort of fishing. Furled leaders, while requiring a floatant to keep them floating, don't sink all that fast either. Mono gets down quicker in general. Also, most of the casting and presentation benefits are negated by adding weighted flies, split shot, and indicators. There are some folks who suggest using crazy lengths of tippet on the end of their furled leader in order to fish deep. This is mostly a case of making something work rather than using the best tool for the job.
There are a variety of materials commonly used to construct furled leaders and the finished leaders will have very different characteristics. However, all furled leaders take advantage of having generally more mass than conventional leaders. It is this extra mass that allows their superior turnover. This leads to the ability to use long tippets. You will no doubt see suggestions to use 4+ feet of a single size of rather light tippet with a furled leader. While you can make 4+ feet of straight 4x or 5x work on the end of a furled leader it is usually better to make at least one step down before adding your final size of tippet. For example, instead of using 4 feet or 4x, use 1 1/2 feet of 2x followed by 2 1/2 feet of 4x. Of course you can vary this arrangement depending on how much slack you want in your presentation.
At this point it may seem like you will be using a lot of tippet. Well that's true, but the durability and longevity of a single furled leader is such that the use of extra tippet is more than made up for by the fact that you will be buying far fewer mono leaders. Additionally, you will use far less heavier leader/tippet material since you will not have to "rebuild" leaders.
To summarize:
Pros
- Very good turnover
- Low Memory and very supple (varies by material used)
- Durability / longevity
Cons
-No real advantage over mono for many nymph or streamer applications
-Generally Poor sink rate and characteristics.
Personally, I use a variety of different leaders in a typical day of fishing. I use mono leaders constructed solely for indicator nymphing, mono leaders constructed for tight line nymphing, and usually a furled leader for dry fly fishing. If I am fishing a small stream I will usually run a furled leader for the majority of the time. But through it all I am not trying to use one type of leader for everything. Rather, I change to a specialized leader for every type of fishing I do. Yes, I spend more time than most changing out my rigs, but I am happier with the results.
Kev