Since I started bottling last year I've been using a mate's steel twin lever capper like this one -
http://www.ebay.co.uk/itm/Crown-Cap...ver-Home-Brew-Beer-HIGH-QUALITY-/390936562888 - then the other week I was in Wilkos and thought it was finally time to get my own, and got one of the ubiquitous black plastic cappers with a magnet holder like this one -
http://www.ebay.co.uk/itm/Youngs-Tw...own-Caps-Home-Brew-Beer-Bottles-/231017775162
I used that one for the first time tonight, and whilst it is lighter and a little more comfortable to use, it feels much more flimsy and seemed to struggle more with some styles of neck. Then I picked up a Wychwood bottle - which I normally just chuck in the recycle bin - that I hadn't noticed when sanitising them. Gave it a go and BANG - broken glass everywhere
Fortunately it was just the neck, so no beer spilt, but with tiny shards all over I had to discard that one
The issue comes down to the design of the clamp section of the capper. In the metal one in the first link there is a rubber grip (the two red 'C' bits at the bottom of the picture) but the black plastic capper just has a metal plate to do the job. This obviously concentrates the force on a much smaller area, so breakages seem more likely to me.
It was a bit of an impulse buy, but I'll put it down to experience and get the first link type for the next session.