I agree with urnidiot too, but not so much with Panj. To me "only to happy to" is neutral - I don't think it necessarily implies that you're not going to (although it certainly could in the appropriate context). "Will you go and see your aunt?" "I'd be only too happy too!" = "certainly!" For me in either use the phrase is slightly quaint. Yes, grammatically it is a strange phrase isn't it!