Firstly I'd like to say that Greg's my favourite character on the show, so this review may be very slightly biased due to it being such a big episode focused on him.
It's about time that Greg got a story devoted to him, and the Old Vegas film-noire touch to the episode couldn't be more of ahomage to him. The episode is different and a breath of fresh air from some of the more disturbing cases in recent weeks and whilst the case was far from the best, the episode appeared more character-driven. I was also partially relieved that it seems that the guys at the crime lab CAN get lucky and aren't useless. xD
The episode isn't perfect however, as whilst it's certainly unique, it is ultimately very predictable which lets it down somewhat. Although I liked the cheesy direction the episode took, reflecting the film-noire style, lines such as "I've loved you since Tuesday" are bound to raise a few eyebrows towards the quality of writing. However many parts are also humorous such as Henry enquiring whether Ellen has a sister.
What this episode excels at is highlighting Nick and Catherine's relationship with Greg. Nick egging him on to get Ellen's number and his later suspicions show the character's friendship, and arguably the best part of the episode, the final scene with Catherine highlights her motherly attitude towards her co-workers, something we haven't seen since Lindsey was a regular.
Whilst it's not a perfect episode, nor the best in the season, if you are a Greg fan, you'll most likely be satisfied with it and even if you're not, the episode is very entertaining and offers a different episode style to CSI.