The only thing that I don't really like about the Yamaha SPX990 is navigating through the screens. I wouldn't necessarily say that it is hard, it just isn't all that user friendly in my opinion. There is definitely a lot that you can do with it in terms of storing your own sounds and all that, but I haven't really delved in that deep with it since I haven't really had to. The manual would be a good idea to have for someone looking to learn all the ins and outs of the unit.
The quality of the effects found on the Yamaha SPX990 is pretty impressive. While perhaps not the warmest sounding effects, for a digital piece of gear it definitely sounds good. The reverb is definitely the best sound on it, but it nice to have the other effects like delay and a variety of modulation effects as well, as you never know when they might come in handy. I've mostly used the SPX990's reverb on vocals, but the effects on it will translate well for pretty much anything that you'd like to use it for. Most of the effects on here are very respectable sounding, but where this makes its money is with the reverb.
While I don't think I'd recommend the Yamaha SPX990 for home studio owners since I wouldn't recommend using outboard multi-effects processors in general, I would recommend it for professionals who are looking to add to their palette of sounds. Of course this could be used in home studios as well, I personally would just invest in some good sounding plug-ins to save the hassle of having outboard in a home studio, as I don't think that this sounds good enough to warrant it. All in all, the Yamaha SPX990 is a very solid multi-effects processor for usages of all types.