London Kings Cross is a fantastic station for spotting, but, I found it very dim and poorly lit so photography was a bit of a problem. Best to go to the end of the platforms to take shots there. I found most of the staff very unhelpful and gruff the day I visited, being told by one member of staff that if I took photographs then Network Rail Security staff were likely to remove me from the station! (I found that rather odd, as on their website I found a piece to say that they welcome rail enthusiasts in all their stations!)
Just find an un-manned barrier and make a dash for it to the end of the platforms (8 and 9 are best because you can access suburban services from there!) and you should be O.K.
Food and drink is plentyful here (take plenty of cash) and all the usual facilities are in the concourse.
Best to travel here by public transport as parking is virtually non-existent.
Plenty of (GNER) HST 43's & 91's and Hull Trains 222's to see on platforms 1 to 8, where 9 to 11 service suburban services with loads of 313's 317's 365's etc.
Started here as a kid, so sentimental value puts it in the ratings for me at least, of 8½/10.