The first lesson I learned is don't judge a film stock by it's ASA/ISO rating. Film stocks have different contrasts and color personalities. Some stocks are more forgiving when the exposure is off. Each one likes to be overexposed a certain amount, but only within a limited range. They respond to color differently, so some work better on people and others on landscapes, or even urban photography. Grain is generally related to ASA/ISO rating, but even with ISO 400 film the grain is generally fine enough if you overexpose the correct amount.
The summary of what I learned about Ektar 100 is that it's a high contrast, highly saturated, fine grain film stock. It's mainly suited to add contrast to landscapes and urban street photography. What it isn't, is a portrait film stock.
Let's start with this example; the bridge was back lit at maybe 10-11am in Washington State. Notice how the shadows of the trees are nearly all black/dark brown? The back lit water is blown to white? This is the type of contrast I typically saw with this particular film stock. Even if you turned up the printer lights while scanning the shadows would just turn muddy brown. A medium quality digital camera would have handled the tree shadows no problem.
Now if we look at color rendition, this stock has a very over-saturated look. Typical of Kodak, the saturation favors red. Anything slightly red or green...trees, dirt, skin, clothing, etc...gets noticeably hyper; which makes sense for a landscape film stock.
As far as under-exposure goes, Ektar 100 is very unforgiving of exposure issues. It very quickly loses information in the shadows and goes muddy brown if you underexpose it. The picture below was under exposed between 1-2 stops and I had to do my best in Photoshop to get it back to the very unspectacular picture you see below - including a large shift in color away from brown/red. The negative just doesn't hold shadow information - which makes sense given the high contrast photos previously shown in this post. A good digital camera at ISO 100 would have been fine here.
Grain is not really a factor with Ektar 100. I was doing high resolution scans on a Nikon Coolscan 5000 (4000dpi, 16-bit) and properly exposed photos didn't have significantly noticeable grain. See the 100% crop below.
What I've determined for myself is that Ektar 100 is a specialty film stock. It's not what I'm looking for. If I was to shoot this film stock again I might try to over-expose by 1/2 to 1-stop and pick my subjects carefully - because it doesn't handle highlights well either. I want a film stock with the latitude of a modern digital camera and a more neutral sense of color, which is part of the reason I shoot digital photos in raw. There's never enough time to perfect a photo on location - which is what this film stock pretty much demands.