I have stayed a couple of times at the Principal London (formerly the Hotel Russell on Russell Square).
A lot of money (and I mean millions of £'s) was spent on its transition from Hotel Russell to Principal and it was very very nice. However it never seemed very busy and even one of the managers said it was quite often 'dead'. I am not sure if Principal's pricing put people off (I was always on a corporate rate paid for by my then employers) but it did seem on the expensive side of things, in comparison to other 'luxury' hotels.
The Principal at York similarly. A lot of money spent on it, which I think was never recouped over the small amount of time Principal operated it.
I look forward to triyng both once under the ihg umbrella.
BM