While Shiny came up with an inspired concept for a game, various aspects of the implementation unfortunately leave much to be desired. While you can control the game with a mouse it is next to impossible as the camera floats behind the character rather than following him tightly. This means that if you wander round a corner the camera takes a second or two to shift the view behind the corner as well. This can be very irritating and although you eventually get used to it (although we'd prefer it if we never had to) it makes using a mouse to control things next to impossible. To make matters worse, Shiny included the now ubiquitous zoom mode (although they did admittedly invent this game feature). Zoom mode requires use of the mouse and mouse movement is just as sloppy and imprecise as it is normally which means you'll pull off no more than one zoomed shot before being rushed by armed Chots a mile off, helplessly trying to aim at the second target with no success. Even once you get used to using keys for movement you'll hit problems. From time to time we let go of a key only to see Bob or some denizen of the Messiah world continue walking along with no input from the user as if their key was stuck down. This led to more deaths than we'd have liked. There are also regular freezes, the odd crash and various other bugs that mar a game that initially showed so much promise and indeed would have been great with better controls and fewer glaring bugs.
Along with the unique gameplay mechanism of possession Shiny have really been promoting their engine technology, and on paper at least their technology called RT-DAT, or Real Time-Deformation and Tessellation, is certainly quite impressive. The easiest way to describe it is LOD on steroids. You can actually set your desired FPS and Messiah will do its best to meet that target regardless of resolution by reducing polygons on characters and levels on the fly, intelligently picking the best areas to drop detail on such as the less noticeable edges of the screen. This means that characters are potentially very attractive with high numbers of polygons allowing greater levels of detail and realism. Note the use of the word "potentially" there, though. The effect of the high poly models is often ruined by the Messiah game engine as characters frequently lose or gain polygons that pop in and out of existence turning potentially very nice characters into continually warping and almost ugly figures. Some of the game areas are quite impressive though despite slightly bland textures, and it is nice that there are very few load times. The game is almost a seamless experience like Half Life was with only brief load times at the odd door thanks to Shiny's use of portal technology.
Sound in Messiah isn't too bad at all. Bob's comments are occasionally amusing, the gunshots sound good and the voice acting on the characters who occasionally comment about your appearance or start talking to each other is pretty well done over all. It won't blow you away, but it won't get on your nerves like the controls and bugs will.