IMHO they are a waste of money although some people like them.

We have a Black and Decker one from Walmart. $69 Cdn. Works great for the last two years.. No inconvenience of cords, gas, etc. No maintenance. A single charge is way more than is needed to do the trimming along our flower beds, sidewalk. fence, etc. It doesn't have the power to to clear out brush but just fine for the city dweller. (Also quiet like a plug in model).

We also use a rechargeable hedge trimmer with the same results. I used to not be a fan of rechargeable tools but they've improved a lot in the last few years. I used to curse rechargeable power drills but now I have two 18v 'Max' brand ones that cost me only $29 each and never touch my expensive Dewalt with a cord anymore. I actually prefer the less expensive models.

We have a Craftsman 24 volt trimmer now going into its third season. It runs about 25 minutes on a charge, and has adequate power for grass and small weeds. It is a little heavy, and requires me to stand in an awkward position, I get a sore back. It tires out at about the same time I do, so it works just fine for a light duty small job trimmer.

Ours is a B&D. Does the job it's designed for. I can't say weight is a problem as the charge doesn't weigh any more than a hanging cord. The comment on the improvement in cordless tools reminds me of the hisory of latex paints. The product had a lot of bugs when first put on the market and turned a lot of people off until they tried it again a decade later.