I wouldn't say that the Focusrite ISA One is hard to use, but there are a lot of features available that one should be aware of. For setting your input level, there is a main fixed level parameter to get your initial level and then a finer trim knob for getting more exact levels. The fixed gain knob also has a corresponding button where you can double the amount of gain used, serving as a pad when not engaged. There is built in phantom power, a phase button, a high pass filter, and controls for assigning your input and impedance. You can also set your headphone and DI levels with separate controls. A manual probably isn't necessary but could be useful if you're looking to use all of the features available here.
The sound of the Focusrite ISA One is super clear and is definitely one of the best sounding Focusrite preamps that I have used. It sounds similar to an API preamp to me, but has a lot more features to work with and is even a bit cleaner than that. It's very transparent and provides a pretty accurate picture of what you're recording, although personally I prefer a thicker sound like you'd hear with a Neve. Still, the ISA One is a great sounding mic preamplifier.
The Focusrite ISA One is a great choice for those looking for a very nice single channel mic preamp with a lot of the amenities that you won't often see in a single channel preamp. The sound is great but the price is the best part of this, as it's one of the most complete preamps that you will find for the price. Definitely worth a test drive for anyone in the market for a single channel preamp for their home studio on the cheap without sacrificing options and quality.