Note: for full disclosure the Eee Pad what I thought I wanted when I first started looking at tabs.
I am writing this review as I believe all of these comparisons to date have been based on box stats and not real world trials. Below I will layout the features that really matter (e.g. I'm not going to review bluetooth, let's make some assumptions here) and how they stack up in a direct comparison.
Weight / Build Quality
Held each tab, A500 in my left, Transformer in my right, you CAN notice the difference, is it significant? Strangely when holding one tab the Transformer feels way lighter, however, when you pick the Acer up again, it feels almost the same. It's hard to explain, go try it yourself.
When holding the Asus you immediately realize the reason for the Acer's extra weight is better build quality. You can literally FEEL the build quality difference. I pushed on the screen of the Asus around the power button and it receeded about 1/8th of an inch! The Acer's screen also flexes, but not nearly as much. Asus build quality is terrible, I would gladly pay the extra weight penalty for the stronger build of the Acer.
If you are a thumb typer, and if you find wide bezels to be an issue, you will notice that the Acer actually has a narrower bezel than the Asus, I couldn't believe the Asus' bezel was wider, but it is!
Looking at the tech. specs each of these tablets should have pretty much the same battery life. The battery capacity of the Asus is 6600 mAH and the Acer is about 80 mAH less, a negligible difference. They run similar processors, however, some have said the IPS panel of the Transformer equates to better battery life. Something however is amiss in the numbers department though... Asus has said 9.5 hours of battery life, however, every review online (and I looked at 6) pegged the Asus' battery life at ~7.5 hours and the Acer's battery life at ~7.5 hours, there is no real world difference here.
Considering the Acer has low power DDR2 RAM while the Asus does not, this may also explain some of the balance as IPS panels have been shown to run less power.
Camera Quality (back camera, who cares about front anyway)
With the update to 3.2 and 1080P recording, the Acer has an AWESOME video camera now which takes great stills too, I was blown away (have you tried it yet? If not, try it now!). This paragraph will be short and sweet, the Asus' camera blows. It takes crappy video, it looks way less clear and crisp and the same is true for stills. Not even in the same league as the Acer's camera now and there is no flash, the Acer dominates in cameras bigtime!
The Asus tries hard, however, it just can't match the Acer. The USB port of the Acer will come in handy at least once in its lifetime, not to mention the ease of adding a keyboard, game controller, etc. Moreover, the HDMI connection on the Acer is micro, while the HDMI on the Asus is mini. Mini HDMI is much harder to find than micro, I had no problem finding a micro HDMI cable, however, mini HDMI cables had to be ordered off the web.
$399 for 16GB, both models, no difference anymore!
The Acer I ran tested on the floor of course utilized Honeycomb 3.1 while the Asus was running Honeycomb 3.2. Even though the Asus has the latest version of Honeycomb, the Acer performs much smoother while scrolling the UI and loading applications. The difference was evident, so much so, this was ultimately the deciding factor for me. The Acer appears to dominate here even with the older software and similar hardware. Some have rumored that the speed increase of the Acer is due to its using LPDDR2 RAM when the Asus uses older DDR2 RAM. I can not confirm this, but benchmarks also showed a slight edge to the Acer.
There has been a lot of discussion of Asus' screen quality vs. Acer's. I went in with the expectation that the Asus would be noticably different (I have perfect eyesight btw). I could not, repeat, could not tell a single damn difference between the two when both on high brightness with XDA Developers loaded in the browser. Perhaps the Asus was slightly "crisper", however, that could be my IPS bias. There is no difference folks, and if you can tell, my hats off to you.
The Acer losses out on glass though. The Asus uses corning name brand Gorilla Glass, while the Acer states it only uses "tempered glass". If you expect to not get a case, or if you are prone to dropping your tablet on its face (most likely you will destroy more than the glass) then you must consider this angle. Gorilla glass is going to help a bit with scratches and perhaps cracks, however, I also noticed it shows more finger prints than the Acer does. *see post #2 apparently the A500 does have gorilla glass!
Screen angles on both are a joke. You can look at the Acer up to 160 degrees, why would you want to? You can look at the Asus up to 179 degrees, but why would you want to? Screen angles are pretty much identical, both are very viewable from the side.
Acer has basic cases and other crap, the Asus of course has the whole battery extending USB-porting keyboard dock. Obviously, Asus destroys Acer here. If you don't already own a laptop or netbook, this may matter to you. However, remember that most of your productivity applications are probably not in Android and if you want to be productive, a tablet no matter what dock it has is not nearly as effective as a laptop.
Another landslide for the Acer, the Dolby sound is the best sound I've ever heard on a mobile device, no joke. I had heads snapping in my living room trying to figure out how such big clear sound was coming out of a tiny little tablet. People will notice. The Asus? Sounds exactly like you'd expect a tablet to sound, like a tin can.
In summary, I ended up buying the Acer A500. With the Asus I could get a bit lighter weight and a keyboard dock that's totally awesome. These were the main benefits the Asus offers over the Acer. As I discovered the lower weight was due to poor construction and materials, the slighter weight of the Asus was no longer a factor. As I didn't need a keyboard, the obvious choice became the Acer due to its superior usability overall.
I hope this helps prospective buyers!