It's pretty much set in stone that the first three seasons and the first movie are the big cut-off point. Depending on who you ask S4 is fair game.
There are occasionally some good episodes in later seasons, but by S6-8 it's pretty much the Krusty Krab/gross monster show.
Watch the second movie. Depending on whether or not you liked that movie's off-the-wall storytelling, you might get some enjoyment out of the creativity of S9-onwards.