At the moment, I'd say it's in the middle (as in, halfway between meh and must have). At the end of the day, Miles B-Sides are, despite what the hardcores might say, still B-Sides. Miles didn't put a fair bit of this stuff onto albums for good reason. Unless you're REALLY big on those fusion albums, not really essential listening.
Having said that, most jazz takes a LONG time to grow on me. I normally have a good idea roughly how I will feel about an album after 3, 4 listens. With jazz, I'd put that at ten listens, sometimes more. So the stuff which I like at the moment, I could end up loving, and the stuff I'm not so keen on... well, yeah.
One thing that I found interesting is that you can hear the band's sound creeping towards the sound they capture on IASW with the preceding sessions*, but they're still not REALLY close right up until the actual sessions that the album tracks came from, and BAM, out of nowhere, there's McLaughlin (from what I understand, they brought him in on a whim) and they're suddenly there.
There's also two tracks at the end of the boxset from the one session that took place between IASW and BB (if I'm right, BB directly followed the stuff on this boxset, and ALL the extras on the BB Complete Sessions boxset are from sessions following the ones for the album itself) and one of those tracks, The Ghetto Walk, is almost perfectly situated between the sound of Silent Way and Bitches Brew. Really interesting track purely because of that.
*Despite the titles, which to me imply you're getting the music just from the sessions of the album named - i.e. the scraps of music they made while making that album - these Miles fusion years Complete Sessions boxsets are really just a way of reducing the masses of stuff Miles produced in the studio during that period, for which there were previously tons of bits and bobs B-Sides albums, into four convenient brackets corresponding to the four studio albums. Which means the focus is really more on delivering the material from the sessions directly leading up to and following each album together WITH that album in one package than it is delivering the scraps of the sessions themselves. The exception is Tribute to Jack Johnson.