Have a Makita as I already had other tools that use the same batteries and haven't had much in the way of complaints. The nice thing is they seem very dedicate to their 18V battery interface, to include using batteries in pairs to effectively have 36V tools. .....and they look to be coming out with a battery backpack that holds 4 batteries and can support either the single or dual battery tools.
Makita aside, the other brands/battery systems are also worth looking at (I also have some Milwaukee tools) but I would recommend looking at the variety of tools that use the same battery, the total run time that can be achieved (either through battery sizes available, ability to operate as a plug-in, or availability of a backpack battery/power source) as the batteries are generally where the cost will start running up.
May also be worth looking at what type of promotions the company runs in the way of reduced prices (personally a big fan of how Makita will run promos that include an extra sets of batteries as I've leveraged that to good advantage).