Miles' Decoy is a great fusion album. If you rate it lower than 5 stars be prepared to rate all 80s fusion albums below it. Why? Because it retains the originality and style Miles has always had, and it hasn't yet moved into "cover tune" territory. Miles allowed his players to put their individual stamp on his music. Jones, Scofield, Foster, Marsalis, etc. All place a great stamp of their own on Miles' Decoy. It has a modern feel to it, and yes, there are synths on it. I like that. The band is performing better than any "fusion" bands of the time, and they are remaining harmonically, rhythmically, and melodically original--unlike most of the Fusion and all of the past-derived "jazz" that was starting to hit the scene at the time. There are metric modulations and interesting changes happening here, unlike funk-influenced "pop-jazz" or "cool Jazz" fusion that was all over the place back then.
I was really just starting to get into Miles Davis when Decoy was out, in the mid-80s. I bought this on cassette when LPs were waning in popularity and cassettes were starting to gain more prominent use. CDs were around but they were a new format. I played this album lots when I bought it and it was my "gateway" Miles Davis CD that got me hooked on Miles. So, I can say with confidence that there is a musical "density" to this music that definitely puts it on par with Miles' other music before and, certainly, after.
The improvisational quality of this music is such that it can withstand repeated listens and still you find something new--that is the consistent quality that all of Miles' best recordings have. So I suggest that Decoy is incredible for you whether or not you're a "completionist" fan of Miles Davis.