I bought the 18v kit with the 5Ah battery. I got this kit so I could also use the battery in the mower I got at the same time.
The lawn is about 15x8m but in an odd shape with obstacles so takes a while to get round it. After this I get abut 20 minutes of run time using the line trimmer.
My initial impressions of both the mower and line trimmer were that they were a bit plastic and felt a delicate due to the lack of weight.
I've given both heaps of crap now and am actually impressed at how well they are holding up.
I recently bought replacement line and did see those dual bladed line heads in store. I almost got them but wasn't entirely convinced at how long they would last with my use as I've got quite a bit of concrete edging and thought it wouldn't take long to wreck one of them. For about the same price you can get a pack with 3 spools of line.
Retrospectively if I had a larger garden I would probably go for the 36v garden tools. This is mainly based on the mower , a bit more power would be better but also they have more features (mulching) and there is a brushless model. The 36v line trimmer also has the Expand-It range that can be added. But in reality the 18v line trimmer is fine and I'm quiet happy with it.
The line trimmer kit I got came with a 3hr charger. There is a faster charger you can get but all the extras soon add-up.