The Lee Collet has a lot going for it - easy and quick to use, no lube needed, gives very concentric sized necks including straightening out of true ones, suits a range of brass thickness and also replicates the effects of a conventional die where expansion is done separately using a mandrel. It is so good in these respects many top US and Canadian long-range precision shooters use a combination of the Lee Collet and a body die (ie a full-length die that doesn't touch the neck) to get the combined benefits of precise shoulder bumping and the Collet's strengths in the neck bit. They work the brass a lot less than conventional dies so work hardening is less and brass life better in some (slack) chambered rifles.
There are potential downsides:
1) as is so often the case, early examples back in the 1980s were really well made. Current ones often have 'tight' sleeves, and collet tines with sharp edges that score the neck deeply. A bit of polishing work is often required to overcome these production failings.
2) some people seem to find them hard to set up in the press and/or think they don't provide 'enough' neck tension - neck tension is usually less than from a standard FL die, but that is often way too much. The hard to set up issue is another result of a poorly made die in some cases where the sliding collet is such a tight fit in the die body that it crushes / telescopes the case shoulder before it slides internally and closes the collet tines on the case-neck.
3) many factory rifles have marginally unconcentric chambers - FLS is always better in such cases and will give more consistent results in terms of groups. (This applies to all forms of neck-sizing not just the collet.)
4) In most cartridges with significant body taper and shallow shoulders, brass 'flows' forwards under firing pressures and due to internal forwards pressures on the unburned front part of the charge during the early part of the burn. If full working pressures are being used - ie in the mid to high 50,000s psi - the shoulder will move forwards noticeably in a single firing. Neck sizing (by whatever method) alone will very quickly see some rounds hard to chamber having become a longitudinal crush fit in the chamber (ie marginal negative headspace). There are three results from this condition. The mechanical chambering and extraction issues as it affects the fired case too; inconsistency between rounds some still having marginal headspace clearance in the chamber, others not; 'negative headspace' in itself usually reduces precision in itself, although occasional rifles turn up that do better with it. I have also seen claims that crush fit-case cartridges produce higher pressures than those that have the optimal 0.001" shoulder to chamber clearance / headspace, but don't know if these claims are backed up by proper testing.
Unless loads are really hot, it is the shoulder movement that causes the problems, not lower case-body expansion. I use Forster Bushing-Bump dies on my long-range 223 and 308 FTR rounds and this is a neck-size + shoulder 'bump' (ie pushed back) job only, the case-body below the shoulder untouched by the die. This works very well and despite running at over 60,000 psi in both cartridges, I get multiple loadings out of the cases and eventually scrap them without ever having done a full/length (body) size. They chamber and extract very easily. (Being custom built rifles they have very concentric chambers of course.) I have experimented with the bump-die with no bushing in (ie shoulder only sized) and a Lee Collet and obtained equally good results and in some other cartridges use a Redding 'body die' and the Lee Collet as a combination.
IF you have a well chambered rifle, good brass, AND run at low pressures for the cartridge, neck-sizing alone can be very effective and the Collet die is one of the best ways of doing this. In years past for short-range range work I did a lot of sizing with the collet alone using a Lee Hand Press and with very mild loads got excellent results and multiple loadings / firings without any need to full-length size. Revisiting the Collet die a year or two back on 260 Rem, I found that with heavier loads (full-pressure but not unduly hot) shoulders moved enough on a single firing to make this method on its own sub-optimal.
