I have various ones and they are great. You don't even need a battery for it to work (obv, it won't work in the dark!) but the kick off it with no battery on a sunny day is quite something, from personal experience!
if you only need a small one (strip grazing, setting up a pen etc) then the wolseley solar bug is brilliant. it's a single unit which comes with its own earth spike, so dead easy to move. I've had one for about 4 years and it has been great.
I've not got one - did consider it, but decided to go for mains powered as it suited our situation the best.
If I had gone for solar, it would be a Hotline, deffo. We've always used this brand, they're good and reliable - and the firm has been based in Newton Abbot, Devon, for ages, and do repairs on site; easy for us as its only a half-hour or so drive! They're always very helpful and I highly recommend them.
I bought one not too long ago off ebay. Very happy with it so far, works when cloudy/dark for a few days as it holds so much charge. I'm told it packs quite a punch too lol!