I did this with Floyd on their Shine On box set. I had all of the albums already, some on MoFi gold CDs or half speed master LPs. I really wanted the disk of early singles especially since it wasn't available elsewhere.
Moral of the story, I got the boxed set, the coveted early singles disc was eventually released separately, and while it is enjoyable and I am a major PF fan, many of those songs are not that great. Basically, I paid about 100 bucks for a disk I wanted, a book and seven albums that I already had. I didn't notice a big improvement in sound and some were not as good as what I previously had.
Sooo, while most of these box sets are pretty with everything all in one place, most of the bonus tracks I find are not worth it. I picked up the Moody Blues Time Traveller set because I didn't have the originals but I would rather buy some new music than get albums that I have already.