I can hire a Mister X, but his stats seem really low. Does he have any pleasant surprises, or is he just a dude with mediocre stats?
They get decent stats when their jobs are all 5's and had some training. You missed out on Kairobot as well, you can hire him after you get several game of the year (GOTY) awards. Mister X is one of the earliest special characters you can hire, his energy bar is usually way above any other employee at that time (meaning he rarely leaves the office).
You can train king and Mister X for less money; in addition, they have more energy.
Yes, Mister X is worth hiring, He is the first special character, If you get 7 Job Changers, You can level him up at all his jobs to get Hardware Engineer and Hacker, When he is fully upgraded, He is recommended to be used for games and consoles and he is really worth keeping.