There are so many good things to say about this episode. Both cases are interesting, well-thought out and unpredictable, which is essential for a show as routine as CSI.
The Sherlock Holmes crime scene is one of the best they've had on the show visually and the actual case itself proved to be a very elaborate case of 'whodunnit' yet the ending is still a little surprise. There's some excellent character development for Greg as a CSI, but also for Grissom and Sara as well as they are placed in mentor roles throughout the episode. I do think one of the best twists is that despite Greg coming to the wrong conclusion, he still passes his proficiency, even if that was inevitable from the beginning.
The other case brought out the more light-hearted side of CSI, shown well through the relationship of Nick and Warrick. The highlight of the case of course being the experiment with "gello-man" and providing the science aspect of the show. Catherine is also seen here as a leader, overlooking the experiment and her snide comments towards Ecklie also provided some humour as well as the appearance of the running "cost of ballistics gel" gag.
However the highlight of the episode to me is the ending, not just because Greg passed but due to the little snippet of the team celebrating. It's one of those family moments which really highlight the relationship of the team and to some extent, the ensemble cast. Unlike many episodes, which end darkly with the perpetrator being caught, this one ends on a much higher note, and provides one of the best feel-good endings in the series.