I run both, with the Excel the lead on the maths, but the paper logbook the official lead containing all signatures and stamps. I put references to them in the Excel logbook.
When I passed my CPL last year, I sent a printout of my Excel logbook to CAA, with each page signed "certified true copy" by me (printed really small, 15 pages for about 1100 hrs!). They accepted it without any trouble and didn't demand to see the paper logbook. This made me happy, because I like keeping my paper logbook locked in my office where it can't get mislaid.
It seems to give nobody any trouble. I can't see me going for just one however - the paper logbook (complete with a natty leather cover) is "official", Excel is both my backup (and separately backed up), and the place all the adding up gets done.
Before I transferred everything to Excel, I used to just photocopy each page and put them in a folder. It worked, but didn't give me all the extra resources of the spreadsheet.
I don't think that any authority cares whether your logbook is "official" or not, they just want the right column headings and totals. Making your own, that suits what you need from your flying records, has always been fine.
G