A decade ago, I used AS5 based on all the online reviews, etc. Like others have noted, performance is no better or worse than others. After awhile, I tried Ceramique which I liked much better. It seemed like a much better long-term solution.
When my last tube of Ceramique ran out last year, I purchased a huge syringe of MX-4 for almost nothing after coupons/rebates/promotions. This stuff is just like Ceramique in that it is 100% non-conductive and is supposed to last 10+ years before needing replacement. Unlike Ceramique, it spreads much easier and is way easier to apply. I'll admit to using too much of this stuff on my CPUs, but it's non-conductive so I don't feel too terribly bad about it.
I even used a huge amount of MX-4 to bridge the 1mm gap between the VRMs and heatsink of my AGP 6800GT when I removed the thermal pad. It worked just fine. The goop stayed in place and didn't wander or get runny. After replacing the MX-4 with an actual thermal pad, it seems like the MX-4 was the better choice and I should have just left things alone.
I've never seen TIM that is easy to clean up. It's always a goopy mess. Don't bother with expensive CPU cleaners; just get yourself a bottle of 91%+ isopropyl alcohol and some cotton swabs. This isn't very difficult to do unless you've made a real mess of things.
15 years ago, I cared about thermal benchmarks regarding different brands of TIM. Now, I'm just looking for stuff that is non-conductive, lasts a long time, and is easy to spread. Both Ceramique and MX-4 meet my needs well. Get MX-4 or Ceramique when its on sale and don't worry about it unless you're a competitive overclocker.