DAH nailed what I wanted to say as I just watched this recently. I gave up on Season 7 and seeing what DAH posted explains why the show's strengths fizzled.
Castle's goofiness needed something solid like Beckett to keep this grounded.
The first few episodes of season 8 were painful and plain stupid. Watch through Season 6 then walk away to leave with good feelings for this show.
It's a fun watch.