Navigating through and using the Yamaha SPX90 will take a little bit of practice since there are a ton of different windows and things that you can do with it, but it is still pretty user friendly and I think that most will be able to catch on rather quickly. It mostly has parameters for scrolling and controlling the different effects, which range all across the board, but also for storing and recalling different presets. The manual is put together very well, as is usually the case with Yamaha products, so more times than not I'd recommend having it around.
The Yamaha SPX90 pretty much has every effect that I can think of, including reverb, delay, modulation effects like chorus, flanger, and vibrato, as well as compression, equalization, and most other types of signal processing that you can think of. All of the effects are pretty clean sounding, but the reverb is definitely the highlight of the unit and is generally what I use it for. Having said this, the other sounds on the SPX90 sound very good as well and can be used in all sorts of different situations.
I don't really know what the availability of the Yamaha SPX90 would be now, but you will definitely still find them in a good amount of different professional recording studios. I also don't know anything about the price, as I've never had a need to seek one out. While I'd probably recommend plug-ins if you're a home studio owner looking for some effects, if you're a professional looking for digital multi-effects outboard gear, the Yamaha SPX90 is an overall great choice...