Too flimsy for my liking.

Too many break when the dog goes full pelt, and even worse the catches break while people walk them on the road and the dog will suddenly lurch after something (cat/car/squirrel).

I prefer long lines, but I put them on the floor (in safe areas of course).

I have no problem at all with Flexis - in parks/fields/secure areas. In fact, I think they are fab in those places.

Recently our trainer showed me how to use a Flexi properly in order to train Recall and now I realise they can be far more useful than I originally thought.

My BIG problem though is when people use Flexis in busy areas, or near roads. I think it puts both the dog and nearby motorists at risk. The locking mechanism can and does fail and for this reason I do not support the use of Flexis outside of secure areas.

As pointed out, a Halti lead isn't really a designer lead! Only reason I mentioned the brand is because I've seen other training leads that aren't such good quality or as soft, I prefer the Halti ones. I think they're about £6-7 for the large ones now so not mega bucks!
Personally I don't like Flexis for several reasons.
First and foremost - security, have heard of several snapping. I know that regular leads can snap too but this is why I like double ended leads that are affixed at more than one point. Also seen too many people using them unlocked on roads where a dog could easily dart out infront of a car, though this is more to do with the owner's lack of common sense than the lead itself - but obviously if the locking mechanism failed, this could happen accidentally. Heard a few cases of the handles slipping out of people's hands too - could be fatal near a road, and I've heard of dogs being terrified by the handles flying towards them as they retract. On a practical POV, it can be very difficult to "reel" the dog in should you need to for its own safety.
Secondly, they often hinder loose lead walking. Used with the lock off and the dog allowed to wander around sniffing in hedges etc, they never allow for a loose lead - there is always that slight tension. I would rather use a double ended lead that I could vary the length of so the dog gets consistancy, which helps with training.
Thirdly, the safety of others. Heard too many horror stories of rope burns, fingers nearly cut off, tangled messes, etc. not just involving the owner and the dog, but others caught in the crossfire of the dog wandering on the end of an extendable. Tape ones tend to be a bit safer than the thin rope ones from what I hear.

I think they can work in some circumstances when used with due care but for a regular walking lead I'd much rather see regular leads used.

You missed my husband's party trick - taking the flexi off the dog, letting go with me holding the end meaning I get pinged by the retracting lead.

http://www.youtube.com/watch?v=U1NyKgKTrxo
this shows the different ways you can use the halti lead

I've got a Spiffy Dog 6-in-1 Multi-Use Dog Lead, but think i might try the halti one.

Thank you for that. I do like the idea of the double lead. I can't really tell from the video, but is one end a handle (wrong word, a loop, wrist strap) with a fastener on?

I think you may have convinced me My only thought is, the thing I like about the flexi is that it retracts when the dog moves towards you. With two dogs, I don't know if I would be quick enough to pick up the slack.

It's a length of lead with trigger clips each end, and several rings down the length. You can use it with one clip on the dog's collar/harness/headcollar and clip the other end to a loop right near the end to form a handle. Otherwise you can clip at different rings on the lead for a shorter lead (e.g. double it over for its shortest length - there's a ring right by the clip that attaches to the dog's collar), clip it around your shoulds, waist, etc. or to another point on the dog (I do one end on headcollar, one end on harness), or even to another dog (I don't fancy trying that one with a total of 75kg of dog though!)
This image shows lots of the different ways you can use it
http://www.canineconcepts.co.uk/ccp5...ing-lead-1.jpg

Top left is the two points of connection - good for safety if the headcollar was to slip off
Top middle is the longest length you can reach, with it clipped to form a handle. You can join more than one together for a longer lead though.
Bottom middle and top right are the shortest length, doubled over.
Bottom left is around the waist - obviously this depends on the size of your waist, I find it can be a bit too loose on me but you can thread it through your belt loops if you wanted. Around my waist with my big dog walking coat works well though.
This photo below shows it at the middle length (and shows the rings along it quite well too)
Edit: Whoops, that URL is banned but if you go to google images and type "halti training lead", it's the 4th image for me, a white GSD(?/retriever?) with just someone's hand visible on the left.

Red Dingo are very good. I bought one in the RSPCA charity shop last week for £4 and it is very comfy as has padded handle. Nice design too.