Every one can be solved, but I often find in the hardest puzzles that you get to a point where there are four squares left that are all interdependent (four cells at the corners of a rectangle on the board, each can be one of 2 numbers).
Then it's tossing a coin territory.
When I get to that stage, I call it a solved puzzle - it's not my fault that the setter fucked up.
If you need help, go here:
http://www.matriciel.co.uk/sudoku/
If you find they are too easy, try Super Sodoku - it's a 4x4x4 grid, in HEX (base 16).