I have given my opinion on various other posts; I'll repeat myself...
I really really regret Apple's decision.
I have gotten Capture One and frankly, am astounded by the quality of the RAW conversions, so much that I am now fidling with old shots, which Aperture was never able to get quite right.
However, what I dearly miss are:
1) Smart Albums; Capture One can't convert them. I use tons of them as my library is composed of over 180,000 frames and I use keywords very extensively
2) Slideshows. There is none in Capture One. I use Apertures' capability to interface with iTunes to do major shows (my pictures are all travel related and with books, slideshows are the best way to share).
So, instead of having one major Catalogue in Capture One (the replica of having one major Aperture Library - which I have), I have catalogues for individual countries or events which are referenced on my external hard drives by continents/events. I then export my Capture One images back to my main Aperture Library (yes, the pictures are way better than Aperture was capable of doing - but I am repeating myself).
Next steps: once I have numerous Catalogues replicating my Aperture Main Library, I'll wait and see what happens in the market. Either Capture One will improve its offering by working on its library management system and introduce Smart Albums and Slideshows, or something will happen with Photos now that Apple is going open architecture; there appears to be some noise comming from Macphun. In the meantime I'll stay away from El Capitan until I get confirmation Aperture can still work with it...