Okay, I finally had some time to sit down and do a review/record some clips with my M-288, sorry for the delay! Chronicle did a good job with Growl vs. Girth and various settings, so I focused more on comparing the MXR with my EBS Octabass directly, with my favorite settings on both pedals.
Build
Both pedals are built very solidly built and I don't worry about having either one on my pedalboard. The knobs on both pedals feel solid and rotate smoothly, the switches have the same stiff "clack" when you step on them and the jacks on both pedals are nice and tight. The EBS is heavier, but the MXR definitely doesn't feel wimpy. I like the finish on the M-288 better, but I like blue and it sparkles.

(finish isn't that important of course, it's really about sound)
Features
By now, almost everyone probably knows these two pedals, but I'll do it anyway... Both pedals are really easy to use. The EBS has two knobs (Normal and Octave) with a Range switch, the MXR has three knobs (Dry, Growl and Girth) with a MID switch. The range switch on the EBS controls the mid voicing, the MID switch on the MXR boosts low/low-mid frequencies (selectable between 400Hz/750Hz) from +4 to 14 decibels.
Sound
I'll start by saying that both pedals have a great sound. Now for a little more in-depth...
EBS - To my ears, the Octabass seems slightly compressed when compared to the MXR. At the same time though, this makes the Octabass seem a bit smoother. The Range switch varies up the voicing of the Octave a bit and the Low setting adds an absolutely HUGE low end sound. I typically leave it on the Mid setting, which seems the most natural when compared to the Octabass's dry sound. The Octabass is better at a "wet only" sound for a bit of synth fun.
MXR - The MXR's MID switch makes a WORLD of difference. Without it, the M-288 has a good beefy octave sound (as expected), but with the MID switch on, it adds an awesome midrange punch that all the difference and really helps cut through the mix. It seems like the wet controls on the MXR aren't meant to be soloed because they seem a little weak on their own.
Tracking - Both pedals got the active/passive bass treatment and seemed to track the same with each. On average, the EBS tracked pretty well down to about A on the E string (an occasional blip here or there, but nothing persistent, you'll hear it in the EBS tracking clip) and started having trouble below that. This isn't
that big of a deal for me since I make a habit of playing higher on the neck when I use octave. The MXR tracked better, consistently tracking down to low F# on the E string. I didn't have any problems with blips like I did on the Octabass.
Verdict
For me, the MXR wins. The MID switch is what really does it for me... It gives the BOD more attitude and cuts through the mix better, but the dry/octave sounds can still blend nicely if you leave it off. You'll hear from the recording that the MXR isn't
too much different than the EBS, but through my rig, it's different enough to have earned it a spot on my board replacing the EBS. It's the MXR's subtle differences that make it a winner. Plus it's blue. And sparkly.
My favorite M-288 setting: Dry @ 5:00, Growl @ 2:00, Girth @ 4:00 with the MID switch on @ 750Hz (of course

)
By the way... Doug Johns was at the Rocky Mountain Bass Slam this weekend, and he was using an M-288. It sounded incredible and he IS incredible.
Hope this all helps!
5sg.
And now, the clips:
MXR M-288 vs. EBS Octabass
(clean, MXR without MID, MXR with MID, EBS)
http://soundclick.com/share?songid=7574511
A note: The groove I played in these clips crosses between the A and D strings, you can hear the frequencies where the MID switch really kicks in even between strings in the third section of this clip.
MXR Tracking
(on E-string, C# (9th fret), down by 1/2 steps)
http://soundclick.com/share?songid=7574522
EBS Tracking
(on E-string, C# (9th fret), down by 1/2 steps)
http://soundclick.com/share?songid=7574518