I started with the Lee Perfect and for the price, it was unbelievably good. What I am saying is that I paid like $15 for this when every other measure was $60+ and this would have been circa 1991 or so. It worked extremely well for the low investment dollars. It's not as good as the other measures, obviously, but it's -FAR- better than being stuck with only dippers or not even dippers.
My next measure was a definite investment for me, at $65 (at the time), the big tall Hornady measure was like adding a Cadillac to my budget load bench. I used it for many years with success but there were frustrations associated with it also.
One day at a gun show I spotted a barely used Lyman 55. It definitely wasn't new but it seemed totally ready to go and for $20, there was simply no possible way to fail.
Not to overstate it, but to this day, and nearly 30 years and documented over 100,000 rounds loaded, this has been the single finest tool that I have ever owned associated with handloading. I've said it before, if the house was going up in flames (okay, bad analogy...) if the house was getting sucked in to a sink hole and I could only grab one thing from my loading room, the Lyman 55 is coming out of that house safely with me.
The design in post #6 above me by Road Clam is accurate in that it is more complex, but in my opinion, THAT is what makes it better (far better) than the Hornady I had been using before. I have never used the RCBS Uniflow that is getting all the raves in this thread, but the design of the RCBS and Hornady is similar -- it is a single chamber adjustable tube that fills almost vertically and is rotated downward to empty.
The Lyman 55 uses a horizontal chamber that has three sizes of adjustment and across absolutely -EVERY- powder that I have ever tried with both, I couldn't find a reason to keep my Hornady for anything except an emergency backup. And shortly after I added a second Lyman 55, I moved the Hornady to a friend that I taught to handload who just really liked it after I let him borrow it while he pondered his choices.
Everyone should be blessed with a measure that they genuinely love and trust, no matter what make or model they end up with. For me, that's always going to be a Lyman 55 and if the last 10 years and 80,000+ rounds of ammo is any indicator, it may very well be this one single unit that I found at a show, used, for $20.