People who cover their planes with cloth and dope need to get with the times! Just kidding but some of the replys to the question are about that rediculous. I've been using Monokote for over 25 years and have lost track of the number of models that it's covered including complex schemes like Hale Wallaces original Steen Skybolt layout. I still use it for the vast majority of my "sport" aircraft where I want the durabilty and I'm not concerned with shine. I've also used most of the other products with good success as well. In my opinion it comes down to personal preference and skill (that should get some good responses). Here is what I have found over the years through personal use...

Monokote: 30+ planes (hence the most experience with it)

Pros: Light weight, large array of colors, color consistancy (roll to roll and year to year), ease of cutting complex designs (wet down on glass to cut), great adhesion, rarely lifts if cared for, most colors do not fade (wines do), has matching paint (Lustrekote) although it's tricky to get a good finish, tensions well over large spans, wrinkle free finishes are a snap due to thin material (especially when darting and working inside corners, lasts for years when done right, price, ease of repair and I have boxes of scrap and extra rolls.

Cons: Easily bubbles when applied on itself, loosens as humidity levels change (effects all coverings since it's the airframe that "moves" with seasonal changes in humidity), not as "user friendly" as other coverings, color smears at edges if applied too hot (clean up with a cloth dampened with lacquer thinner), has to be occassionally "reshrunk" with a heat gun.

Conclusion: Great product with its own quirks.

Ultracote: Have covered 3 or 4 planes with it.

Pros: Goes on great, low level of outgassing keeps bubbles to a minimum when covering over itself.

Cons: Number of avaiablable colors, weight can be a factor, poor edge adhesion and occassional lifting, large spans can sag and need to be reshrunk, no matching paints (sorry covering materials do not belong on plastic or fiberglass parts), significant thickness build up can occur when overlapping, thickness of material promotes edge lifting (gets caught).

Conclusion: Great product with its own quirks.

21st Century: 2 scale planes covered

Pros: Great scale low gloss apperance, great adhesion when applied at the right temperature, good scale color selection, complex graphics hold their shape due to cloth substrate
Cons: Heavy, thickness promotes edges gettign caught and pulling up, difficult to properly tension large spans
Conclusion: Uh...great product with its own quirks.

In the end the level of preparation, individual skill and patience will determine the outcome regardless of the product. Just like painting. I know guys who can glass and paint a model and get better results (weight and finish) than iron on coverings but I'm not one of them. And although I miss the dope and silkspan days I've already killed enough brain cells doing that and can't afford to lose anymore. It's all aobut what you are comfortable with. Experiment and stick with it.