Basically as with so many things in life - you get what you pay for. I had a cheap strimmer which lasted 3 months and when I used it again...it was knackered! I then bought a decent electric one (cost me about £50) and I've had it 3 years and its absolutely spot on.
On the issue of electric vs petrol - it all depends how much you've got to strim (as another forumer previously said) - a friend of mine moved into a new house with a huge back garden.....all of which was just about head height with overgrown grass!! We managed to hire a big petrol strimmer which you effectively strap to your body with a harness and what could have been a two day job - we'd finished in 5 hours. Just don't be a numpty like him and have a fire with all the garden rubbish and because it didn't take hold immediately he threw petrol on it - and got blow back and took his eyebrows off....

Mick