I think it depends on what your needs are.  I've had my SP4 i5 since it came out.  I don't see anything about the 5 or 6 that would make me want to upgrade.  The battery life has improved, but I've never found myself needing more than the 4 hours or so that I get.  I only use my SP4 for travel and miscellaneous use.  I have desktops at home and work.