I've got 18V DeWalt for most of my kit and it's all anywhere from 3-5 years old, has taken plenty of abuse and is still going strong. My only complaint about it is it's a bit heavy due to the Nickel Cadmium batteries and the tools themselves are a bit bulky. I've also got 10.8V Bosch impact drive, screwdriver and 36V SDS. These are good bits of kit too but I find the 10.8V stuff is a little underpowered and (all) the Lithium Ion batteries hate the cold. If it's a cold day and the tools have been stored in the van overnight you can forget about using it until the batteries have been warmed up or atleast charged.
I have recently bought myself a Milwaukee 12V multitool which I couldn't be without now. Amazing piece of kit in power and battery life plus Milwaukee have engineered their batteries to work in cold weather so no worries about flat batteries on winter mornings. The 12V gear is surprisingly powerful too, I played with some of it at Toolfair last year and was amazed when I put a 8mm drill bit in to a concrete curb stone with a tiny combi drill that weighed next to nothing. The tools come with a 3 year guarantee and the Red Lithium batteries 2 year (if registered online). I'm looking to buy a 12V impact driver and drill before the end of this financial year to replace the 10.8V Bosch and will likely do the same with my 18V combi when the time comes. Anglia tool centre are doing a 18V combi and impact drive kit with 3 batteries for £400 at the minute too.
Milwaukee 18V kit