Using the Yamaha Motif-Rack ES is a bit more difficult than using a full Motif keyboard since it's so compact, but it's still not too hard to navigate through sounds here and all that. I haven't used the Motif-Rack ES for it's in depth features, using it as just a sound module with an external MIDI keyboard. There's definitely a good amount that can be done with it that I'm sure is more in depth that I haven't used it for as of yet. A manual is definitely a good thing to have if you're looking to learn everything possible with the Yamaha Motif-Rack ES.
The sounds on the Yamaha Motif-Rack ES are awesome across the board. They're the same exact sounds that you'll find in the keyboard version of the Motif ES, which range from traditional acoustic piano sounds to emulations of acoustic instruments, to synth sounds and beyond. It also has drum kits, effects, and pretty much everything you'd expect a multi-functional workstation to have. All of the sounds are pretty realistic, with some being more so than others, but for the most part this is a top notch collection of sounds.
If you're a keyboard player who has a rack of modules and a MIDI keyboard, you really can't go wrong with the Yamaha Motif-Rack ES as your main module. It's not going to give you the most realistic analog keyboard sounds, but for just about everything besides this, it's top tier. If you already have a MIDI keyboard this is a great way to expand your palette of sounds. It's cheaper than buying a Motif keyboard, but in some cases it's probably worth it to go for the full keyboard. The Yamaha Motif-Rack ES is definitely a nice option to have and is one that should be considered by anyone looking for an all purpose MIDI rack sound module.