I went to some island-themed buffet at the Rio. It was above average, but also just as pricey as Spice Buffet at then-Aladdin (which is tops) so I'd recommend against it.
Penn & Teller is also above average, but again, waaay to much money (I think I paid $85) to make me happy about seeing it.
I've never gambled there.
Edit for clarification: I don't think Spice Buffet is the best buffet in town. In terms of pure quality, I'd vote (1) Bellagio, (2), Wynn, (tie for 3) Spice and Mandalay Bay. In terms of price, I'd say (tie 1) Bellagio and Wynn, (tie 3) Spice and Mandalay. I think Rio is below Spice in quality, but as expensive, so I've only been there once (same with the Wynn, whose price and location just can't compete with the Bellagio). In the interest of full disclosure, I'll also add that I'm aware my opinion of the Wynn's quality is a statistical outlier - all my friends thought it was Excalibur-level.