I have used the Arista E-6 processing kit twice, and processed multiple rolls both times. I developed Velvia 50 (135), Kodak E100VS (220), and Astia RAP 100F (120). This is all within the past two months.
The hardest parts are: temperature control, and timing.
The second time I used the kit *, I forgot to add developing time for the 2nd and 3rd rolls, and they both came out too dark. I am sure that if I'd added the correct time, they would have come out much better.
This past weekend I did some Black and White processing, and effort-wise they are about the same. Black and White even takes more steps than the Arista E-6 kit.
* Actually, there's more to this story. I used the first kit at home, and got good results. So, I had another kit sent to my sister's house (in another state), and went there on vacation. She doesn't know anything at all about film photography. When I got there, I handed her a Yashica 24 TLR, and a roll of Arista 100F. I walked her through the whole process, from loading the film into the camera through taking pictures (I metered for her), taking the film out of the camera and putting it onto the processing reel in a changing bag (and into the processing tank), mixing the chemicals (I handled the temperature control), and then processing the film herself. When the pictures came out she was ecstatic. Big smile. Her roll came out fine. It was when I re-used the chemicals the next day, and didn't add time to account for exhaution, that the slides came out too dark.
So, if my sister can do it, being a brand new person to the whole process, you can see that it really isn't that hard.