If your dog has got the idea of 'heel' pretty well already or generally has no issues walking by your side then an extendible lead will be useful. I like it because when we get into fields or large open areas where really she could be off the lead but other dogs, roads, horses, campsites are near by the extendibility will allow her more freedom than other leads while still maintaining control. I have counted now and there have been 9 times where she been on the extendible lead and bolted towards a road, horse, dog, person, football game, cliff face and i have gone "thank god you were still on your lead"
HOWEVER, i would not adivse having this as your first and only lead. It was mine as it seemed the obvious choice and Daisy did NOT take kindly to the shorter lead near traffic or in towns, not atall. She knows it extends and wants it to be. If your dog likes to pull also, i feel that this extendible lead stuff reaffirms that the dog is allowed to set the pace and distance from you. Something that will not be learned from a static lead.
I have always been told that 2 fingers should fit snuggly underneath the collor but this is often to loose for a dog that REALLY wants to go somewhere and your lead is not letting it, Daisy has slipped her collor several times before and its scary. I advise you use the collor for holding her ID tag and for just something to grab while training, petting, restraining ect and using a "choke collor" (horrible word) to attach a lead to her/him. THis is only because such collors are very fast to get on and off and is very hard for a dog to slip such a collor unless they are calm and right next to you...a dog in such a state of mind is unlikely to be wanting to get off the collor in the first place.
P.S - If you do go with the extendble lead option, chances are itll be a string. Pay attention every week to the state of the string especially nearer the dog end as they can and will chew through it as part of a long term escape plan. Also, the string is often tied to the clip at the end and enough pulling will cause this knot to loosen and release eventually (learnt that the hard way) Try and soak the lead and catch (not the whole lead) in water before going out. Everytime your dog pulls on it, it will make the knot tighter and tigher ! Still check it regulaly though.
Also, DEFINATELY get one with a locking mechanism on it. So that you may set the length and then lock it. Daisy is VERY scared of Lorries that drive past us and it is essential that she is on the shortest possible lead while walking next to a road. My locking mechanism is broken and it will KILL your thumb after 2-3 hours or walking knowing that if your thumb lifts, your dog could be ran over...simple as !
Hope i helped