Well, I've had it over the weekend so I can probably give initial impressions. Probably over time they may change.
First thing, and everyone says this, it is large. Barely fits in my rack.
My girlfriend and I immediately noticed that the surround speakers are more audible. But that could've been due to the amplifier upgrade since my surrounds had previously been powered by the Onkyo whereas the other speakers had separate amplification. So I'm not sure.
I feel the center channel dialogue is much clearer. I've always complained about the polk audio LSiC, but after this change it is clear and I find myself having the overall volume much lower just because now I can hear everything being said, easily. I had an M200 hooked up to the center at one point and still had problems. So that makes me feel it was an upstream problem, either cabling or my AVR. Previously I had to bump up the center channel higher than the mains where now they're all equal yet I can still hear the dialogue great. Maybe this is due to a single multi-channel amp or maybe the better processing and the LSiC was just revealing all imperfections.
I also seem to be able to hear surround effects more when watching regular TV. It seemed like before most of my TV channels were all collapsed to the center speaker whereas now I actually hear surround effects.
I watched a Coldplay concert DVD I use as reference material. I noticed just slightly more detail such as hearing the pick pluck a string rather than only hearing the note. I would say also that with this DVD my Onkyo sounded better with Dolby Digital rather than DTS, which seemed odd to me. Now, the DTS sounds as good, or better.
I think the greater amount of crossover customization has allowed my center and surrounds to perform more to their potential also. Just a more dramatic and realistic experience.
The remote is very similar to my Onkyo remote, so there was hardly any learning curve for my girlfriend. I only wish it was a learning remote so I could program my Cambridge Audio CDP into it. That's annoying, so I might have to buy a new universal. But I knew that it wasn’t learning from the get-go. Odd about the remote is the power button doesn't turn it on, only off. Seems they should've named it Off or Standby. To turn on the 990, you have to choose an input button like CD or DVD.
There are some things I'm not used to about it. On inputs that should have no video, when I push the volume button (and the volume graph pops up onscreen), my TV flashes white. This didn't happen with the Onkyo. What I think is happening is that the 990 actually completely turns off any signal in such a situation and then turns on the signal to OSD only when needed. I think this because, an "Aux" window pop-up on my TV shows (native to the TV) and it usually only shows when there is absolutely no input. But I think my Onkyo had a signal always going to the TV, just maybe a black screen.
The 7.1 analog-in still uses the crossover whereas my Onkyo didn't. The only way to disable this would be to set all speakers to large in the 990.
Night-mode doesn't seem to work in DTS, only Dolby Digital.
Even though the CD input has no video associated, it still displays on the TV whatever the last video signal the 990 was processing. So if your VCR was left on, you'll see it. But if no signal is present, it'll turn off the signal to the TV all together.
They're working now on a firmware update to include various things, including the bass management issues you may or may not have heard about. I recommend reading the forums to get up-to-date regarding the bass management if you are interested in the 990. Let me know if interested and I can briefly type up some things I noticed about it.

Overall, I do like it. Sound quality is great but I don't know exactly if that is the 990 or 7500. Probably both actually. But I am still in that mode of getting used to it and tweaking it. And maybe with the firmware update, I might like it much, much more. We'll see. I also had tubes in the mix before where now I don't, so I'm still trying to get used to that. I would say overall I feel I got my money's worth and it'll even be better after the firmware update.