I've left my bike there plenty of times and never had any issues. I'll be using the train again more frequently from April.
Bike parking used to be on the under capacity racks outside Ryries (and often the nearby railings). The new covered racks nearer the station 'feel' much safer, but I suspect it would make little difference to a bike theif.
The only time anyone has ever interfered with my bike is to leave a Spokes bulletin!
I did once leave my cycle computer on the bike and worried about it all day, but it was there when I got back. Quite often see bikes with lights left on them.
I use a fairly standard U lock, with an additional looped cable around the front wheel.
My main security features are probably:
1) Other bikes look easier to steal.
2) Other bikes look more desirable.
On the 2nd point, my 'station' bike really isn't very desirable.
I arrive just before 8am and have always managed to get a space on the covered racks. I suspect it could fill up later.