Well perhaps Maxwell has learned from its past, because they just released a brand new plugin for SU. It's so streamlined into the software, infact there isn't even an install. Just a maxwell.rb file with a folder that gets placed in the plugin folders and that's it. If that wasn't good enough, they provide it for free, which allows you to render up to 800px. Now if you want to render larger images, the licensed version is nothing to kill your wallet at $95!
The settings that really sold this to me were the materials. They are so simple, and they are almost a seamless 1:1 with what you have already created in SU. So if you tweak a material in SU it gets tweaked in Maxwells material browser. So Maxwell just has the same materials that you already created, with some extra settings. You can change the materials reflection type to tell it to render as metal, lacquer, velvet, light emitting, satin, etc. But what is really clever is that MW automatically creates a grey-scale bump map for your texture map....do I hear a "sweet". It gives you options to invert. What is even cooler is if you change the brightness or color on the map in the MW settings, it changes them in your SU material settings too. Of course if you really want to get fancy, you can unlink the MW texture from SU, but it's just a toggle and you can always turn it back on at any time!
The camera settings are what you would think they are. There are parameters to specifiy the render size. Then there are the camera settings themselves. Much like mental ray in 3dsmax, you can specify your fstop, shutter and ISO, or just your EV value. The cool thing here is that changing your fstop affects your amount of DOF just like a real camera. Also you can choose if you want the focus to work manually, or semi-automatic, or totally automatic, based on the scene objects and distances.
The next settings are the environment or lighting settings. The first option is to choose your type of skylight. You can use the default sky dome, the physical sky (which is linked to your sun angle from SU), or a simple hdr map of your choice. Then you have the option to turn the sun on or off along with other atmosphere settings like turbidity, ozone, etc.
Then, much like vray, you can toggle a ground plane on and off as well.
The last settings I'm going to cover are some custom settings. You can tell MW to use just front or both front and back faced materials. Also how it will respect hidden layers. There is also a material override option for simple testing. Then there is a section for color management. you can tell it to use sRGB, or the myriad of color options. The burn tells MW what to do with highlights, and then there is gamma.
The last section, simulens, I haven't played with yet, but have a hunch that it's really cool. I'm guessing devignetting is simply vignetting in your rendering. There are options to add aperture maps, which i'm guessing control the way DOF creates its bokeh effect. But these are all photo-phenomenon tools that will add realism to the rendering.
That's pretty much it. I was really amazed at how simple and robust the UI was. I'm not sure if its because my computer is any faster but I could guess that the rendering algorithm is faster at computing the data. It seemed that my renders were going quite quick, and gives iray a run for its money. Where some of my scenes wouldn't fit into memory for iray in 3dsmax, maxwell in SU was able to handle them fine. I have a feeling it has to do with SU's small memory footprint, then again, there may be some wizardry coming from the Spaniards!
20min - Still some speckling as I had a huge copper cylinder casting caustics, but not bad
UPDATE (2012-02-02): So just recently downloaded the latest Maxwell for SU, and they did make some revisions in their software. Unfortunately the software now requires an .exe install, and is a little larger, but hopefully that will only make it faster. Also it now requires Microsoft Silverlight, but that's not so bad.