I own a late 2008 unibody 15" MacBook Pro running OSX 10.8.2. I chose the UHS-I ExpressCard reader because I've never used my Express34 Card Slot and I'd rather transfer photos directly from a card than from my camera body via a USB connection. Not only is this more convenient (no cables), it's faster.

======SETUP=======
Setup was a breeze--mostly.
Simply go to Transcend's website, find the driver, and install it.

But wait.

If you're running OSX 10.8 Mountain Lion, you'll be stopped by Gatekeeper, which only allows you to run applications directly from the App Store or from Identified Developers. After some searching, I found you can disable Gatekeeper and allow your machine to install the driver. Just go under System Preferences --> Security and Privacy and check the "Anywhere" button at the bottom. I'd recommend setting this back to its default after the installation is complete. Now that Gatekeeper is temporarily disabled, you can run the driver installation. When it's complete, it will want to restart your computer.

After a restart, you can click the card into place. After that, you're ready to go.

Some have said that this card reader causes their Macs to crash. I haven't experienced this yet. If I do, I'll edit this post.

=======Physical Size, Build Quality======
The card was smaller than I imagined it would be. But then again, I'm used to the larger cards that used to be the standard about 8-10 years ago. On a MacBook Pro, the card goes into the slot with minimal effort. You'll hear a nice solid click when it's mounted in place. To remove it, simply give it a push, and it will click back out again. If you're used to using an SD card (which, you should if you're buying this), the mechanism operates in very much the same way. I'm very happy that the card does not protrude from my MBP at all. It sits flush with the rest of the notebook body. This way, I can pop it in and out of my laptop bag without the card snagging. I've attached a photo so you can see what it looks like installed.

=======Use & Performance========
Once the card is mounted in the card slot and you've installed the driver, it's ready to go. Simply plug in an SD/SDHC/SDXC card and you can access it just like any other card reader. You'll notice that there is now a bold rectangle icon on the top Apple menu bar. The only purpose this serves is to let you know the card is in place. You can click on it to power the card off, but that's the only thing it does. It would be nice if I could get rid of this, or if it had a few more options, like to safely remove (eject) the card, but this is only a minor complaint. You can always eject the card just like any other flash storage.

In practice, the card reader is pretty fast. I notice a significant difference in transfer speeds compared to hooking my DSLR up via a USB cable. I have a SanDisk Extreme Pro 16GB UHS-I (95MB/s) card and decided to run some speed tests to find out how quick the card reader really was (at the time of this review, this is the fastest SD card available). I was surprised to find out that it was writing at about 65MB/s and reading at 75MB/s. Obviously I'm not getting SanDisk's claimed 95MB/s. To be honest, I'm not sure whether this is a limitation of the SD card, or the card reader itself. I've seen other people with the same card get higher speeds, but I'm happy with the read speed. I imported 25 raw photos (~22MB each) recently and it didn't take more than 7 seconds to finish the import. Not bad!

=======Summary | Pros-Cons=======

-----PROs-----
-Works Great!
-Works... On a Mac!
-Mounts flush with the laptop. Does not protrude.
-Installation was quick.
-It's fast.... enough.
-Good build quality.
-Reliable.

-----CONS-----
-Some better documentation, PLEASE. Seriously: There's 4x more Transcend promotional garbage in the packaging than instructions. In fact, there are almost no instructions other than "Go to our website for the driver before use."
-I'm not getting the 95MB/s claimed performance from my SDHC card. (Could be a SanDisk limitation though)
-Had to disable Apple's Gatekeeper to install the driver. A minor inconvenience, but why is Transcend (a major manufacturer) not on Apple's "trusted" list??
-Annoying single-function icon always present on the menu bar. More functionality/options would be nice.
-Packaged in stupid "frustration" packaging. Honestly. It practically takes industrial scissors to get the packaging apart.
-I had to hunt for the driver a bit. It's listed under "accessories" and not "card readers"; Go figure.

All in all, I'm pleased. It's a simple card reader, it's fast, it works great, and setup was a breeze. I WOULD recommend this to a friend.