This episode was incedibly well done.. I honestly have very few complaints... but first, one of the biggest positives for me..
SpongeBob and Patrick's friendship: This was done INCREDIBLY well done.. this how both characters should consistently act. SpongeBob's character in particular has been majorly improved since the second movie, but in this episode, Patrick is the star. This had me quite scared, considering how inconsistant his character has been, but he was done incredibly well here. I truthfully expected the fame to go directly to his head, but it didn't, which honestly impressed me. They REALLY felt like best friends here, and it was easy to sympathise with both SpongeBob and Patrick.
Other noteable mentions:
The way they got around Plankton was both clever AND funny, and I really didn't expect them to do it in that way. Although I'm sure Plankton could've reverse engineered the patty, and figured out the formula with some tweaking, but whatever.
Squidward's character in general... THIS is how he should be, for the most part. In some recent episodes (LiBB and Whirlybrains come to mind), he's just felt completely out of character, but he felt incedibly natural here.
Honestly, I had little to no expectations for this episode of positivity, and this just blew me away. I really thought my interest in SpongeBob, but this proved me UNBELIEVEABLY wrong, and I'm so glad that it did.. bravo crew, bravo.
EDIT: Noticed people saying it lacked in gags... I think this worked to the show's advantage. The newer episodes, in my eyes, have really lacked in plot, and spend far too much time on jokes, so this it was refreshing to see them pay more attention to the characters, and not make constant jokes. And the jokes that it does have, are consistently funny (minus the "square box" joke, which I just found to be meh).
And yes, the plot is similar to "Selling Out", but personally, I always considered that episode to be pretty meh; I really think this episode pulled off the plot far better.