It's great, no, terrific mulch and I like the smell of it. Using it for a packing to overwinter tubers which were dug up might cause a problem of rot due to its moisture content. Note, my experience with dahlias commenced this past spring, so I am speaking in general.

Be aware that it is illegal to take leaf mould from woodland belonging to someone else. Assuming that you can get it legally then leafmould is the BEST stuff you can use in your garden. Make sure that it is dry before using it to store tubers over the winter.