I am performing it in 3 weeks, it's very tricky in lots of places. Variation 15 very nasty, and variation 19- end very hard. It's managable, but lots of nasty bits. The leaps are evil in the last variation.
I think it might be harder than the Brahms,(I started work on Brahms 2 days ago so still not sure) It all depends of hand size as well, I can stretch a 10th confortably, and found some of the Rach hard to get hands around some of the big fast chords, but it did get there, just needed a lot of work. The big flamboyant stuff tends to be easier than the bits you don;t expect to be hard. For example.. The big cadenza variation 11 if my memory severs me correctly is not to hard at all, yet some of the slower ones are very awkward. Variation 15 is a total nightmare, it very very hard to get going, and it realyl needs to be kept on top of.
I dunno how to rate it in terms of difficulty on the whole. I started learning it in January and had the notes finished by May. It's nice to do because you feel like you've accomplished something at the end of each variation, unlike another concerto where the difficulties seem to come at you and the end is no where no be seen. Have a read thorugh when you get a score, and don;t be out off by some bits if you struggle, a lot of takes slow work, when I started on the last variation, I still couldn;t play it at the slowest speed on the metronome, but with lots of work it comes quickly.
Topic: Rachmaninoff - Rapsody on a theme by Paganini