Originally Posted by Longfemur
I rode a bike equipped with a Huret Allvit for well over 20 years. I never had any problems with it - and I rode a lot in those days. It was a basic, solid derailleur which was found on a great many bikes like Raleigh Records, etc. Not high end, but if I had a bike now with one, I would want to keep it on there if it works after adjustment. At the time many Allvits were on bikes, Suntour was becoming more and more common, but as I recall it, at the time, most were on cheaper bikes than the Huret's. For example, the Raleighs (the genuine English ones, that is) had Allvits, but the cheaper Robin Hoods had Suntour. Like I said, personally, I would not change it if it works (and it probably does unless it was damaged, because they were pretty tough derailleurs).
The Alvit was used because it was cheap, relatively durable, and there weren't other low-priced alternatives - not because it shifted well. Sun Tour did start out on the budget bikes, but after a few years, people recognized their superiority, and the Alvit went the way of the buggy whip. If you're into leisurely riding, there wouldn't be any problem with the Alvit. If you care at all about quickly shifting gears reliably, it is a disappointment.
Fortunately, I never had to use one while growing up. My experience comes from one that I completely overhauled about 30 months ago. Yes, it shifted, and would eventually get in the desired gear, but even after a reasonable period of getting used to it, I'd still have to look back to make sure what gear I was in. It did work, but I didn't see the point of subjecting myself to it when something much better was readily available.