Funny you should mention that. About a week ago, I exchanged messages on here with another contributor about the problems he was having with a Black and Decker model (can't remember the model number). He wound up (no pun intended) taking it back and telling the shop where to put it (it didn't sound like that where to put it wasn't going to be the display shelf either).
Any way, I've had a gas powered
Stihl unit here in the U.S. for some time that I had considerable difficulty with as you mention, the string feed. The unit was dependable otherwise so I spent about $12 (USD of course) and bought an exchange head that uses three nylon blades. They still wear down but last considerably longer than even a spool of string and only takes a minute or so to change worn blades...Seen here:
http://www.cmdproducts.com/Grass%20Gator%20360 0.html
Be sure not to hit your toe with them, however... bye-bye toe!
Looks like this: