I know this is an older thread, but I had some input on my experience with one of these too.
Short:
Pros - Works, replaces the commonly non-working clock.
Cons - Wrong span, minor appearance differences, almost-ill fitting, inaccurate by as much as 500-600 RPM in my tests.
Long:
I just installed one of these to clean up the interior a bit. I had an older 3 3/4" Auto Meter Sport Comp tach mounted in front of where the factory clock was. Needless to say, I'm kind of disappointed myself with the reproduction. I noticed it was kind of strange looking...Then I realized that it was an 8,000 RPM tach, instead of the 7,000...but I figured whatever...No biggie.
Most of it looks like it was made in an CAD plastic modeling machine. The mounting baseplate was plastic, instead of the steel, which wasn't a major deal, but the screws are short to begin with and it was quite a bit thicker. Not to mention, same problem...Holes didn't quite line up either. [sigh]
It looks pretty good mounted. The needle is flat...Not three-dimensional like the factory ones, which kind of stinks for the dash light not reflecting off of it at night. But I can look past that I suppose.
I had to fight it getting a bulb socket in there for one of the dash lights. Super tight fit...Did eventually get it though.
The green filter for the right turn signal indicator was disappointing, it's not nearly as bright as the factory one, so kind of hard to see in the daytime, and on top of that a little different color green...But what the heck, I know it's on, and I'm not looking for concourse restoration here. So I'll let that slide...Again.
The 12v and tach connections on the back were reasonable. Looks like they held the whole tach together that way too. The 12v connection stud was pretty long and awfully close to the steering column with the cluster back in the dash. Just made it, but could possibly be a potential for problems if care isn't taken.
Started it up, and it works. Looks a little bit off, but I try not to read too much into it. Sure enough after I warm it up, I give it a full WOT run through the gears...What do I have? AT LEAST 500 RPM OFF.
Explanation: Numerous runs with the MSD 6AL and a 6600 limit. I'd call it accurate with the Auto Meter, right about 6550-6650 it starts breaking up like it's supposed to and doing it consistently. Awesome! New tach? Shows 7000-7100 before it starts breaking up. That's quite a bit off.
This isn't bad if you want something to look at for just cruising around town, or something to take place of the clock in general, but as for function and fit? I'd say the bad outweighed the good on this one for me. Maybe they'll update it and come out with something better, or maybe other companies out there have something a little nicer.