I have been with Sky on the unlimited BB for almost 18 months.
At first it was rock solid, although I could only get about 4meg download speeds. I was happy because it was stable as anything.
Roll on to August this year and I get an email from them saying that they are going to increase the speed of my BB product

Start of September, cue random disconnects and modem appearing to be freezing up.
Read about DLM, so called them up. Had to go through the usual stuff, test socket, etc. No better, so transferred me to Tier 2 support.
Was told that my SNR down was dropping below 6 from about 10 and this was causing problems.
They said that they had altered some settings and to give it a couple of days.
Did a couple of speedtests over the next few days with maximum connection / download speed of about 1.5mb

I am still getting problems now, but have since cancelled my Sky package and switched to Virgin an their 50meg BB.
Sky have been pretty good on the CS side when I speak to someone from the UK, but the deal I have been offered for the Virgin package is pretty much unbeatable (have a friend who works for them and has got me "mates rates", so it works out a 1/3 cheaper than my comparable Sky package)