I put in roughly 3 hours and really loving the game. Graphics look great, if a little grainy (on One X). I have not had any performance issues, but can say that slight pausing when accessing the map is there though. I have not had any texture pop in issues though as reported by some on the PRO. I am taking the game slowly looking in every corner, stopping to read each collectible etc. A tip, I suggest turning ON the subtitles, when those odd sounding voices start, it is a little hard to understand them and with sub-titles on, you can read what they are saying/whispering etc.
I got to the point where you get the Level 1 key and you fight a boss...will not mention name or anything for spoiler purposes, but it has been a hell of a ride so far. It is a complete mind fuck type of game though. In the early part when you get into the building there is a part where you go through, but once you get back to where you started, the area that you just were in is gone and it just is a bit freaky. Gun play and having my first power is really great so far. Some of the collectibles are great reads and do watch the videos again from the menu as they are full screen and look better imho opinion.
I have not come across any side missions as of yet, but am looking at searching every room I come across. As mentioned I just got the Level 1 key so not sure if I missed anything or if they are later in the game. Overall, with 3 hours in it is fantastic so far.