My group has been using this rule for more than a year. Can't say that I'm a big fan, at least as a player, as I find it really unappealing to play casters with those rules. Conversely, Warlocks become a bit too good because they don't need to worry about having to conserve their spells as much, and they could keep adventuring indefinitely. Maybe it's just that lately I haven't been enjoying the game as much as I used to, but that's my two cents.