Like the other reviewers have said, Allerton is totally beautiful. Who had any idea that this was out here in Monticello? Allerton is now owned by the University of Illinois and is used as an all-inclusive banquet center for whatever needs you have.
Our friends got married here in august and it was spectacular. The Mansion has been converted to a small hotel with a good amount of guest rooms. The suites are gorgeous and the smaller guest rooms are more than adequate. The mansion itself has two dozen or so rooms and then the smaller houses on the property have the rest of the rooms (presumably old quarters for housekeepers and such).
The mansion puts on a really great show and the reception was lovely. You have your choice of styles, decorations, etc. and they set everything up. The grounds are also amazingly enormous and beautiful. There's about 10 different places on the grounds to get married. Our friend chose the Sunken Garden and it was beautiful. The space was vast, beautiful and totally serene - great venue for a lovely ceremony.