There are many downsides, and very few upsides. The change to the 360 is due to VMs takeover by Liberty Global as the 360 software is developed in-house whereas the TiVo software on the V6 is licensed from TiVo. The 360 is missing many features that are present on the V6, many of which are TiVo patents so are unlikely to appear in the future.
Some of the missing features are as follows; Wishlists (where you can set the box to record programs on certain subjects or featuring certain people), undelete, series link+, series link manager, suggestions, setting of series links for programs not in the current TV guide, ethernet control, keyboard support via USB, switching between tuners with constant buffering, skip-back on fast-forward, quickplay, default recording options, cancelling individual recordings from a series link, multi-channel series links, radio channels allowing recording of radio, separation of recordings between boxes, watching recordings when internet is not available and probably a few others I've forgotten.
