I have a PS90 and wasn't too thrilled with the price. It was about $400 over the wholesale price. All it came with was: 30rd mag, PS90, owner's manual, and a gun lock.
FN didn't even include a 50rd mag, extra mags, or maybe some extra goodies ala Glock or Springfield. FN doesn't even sell 50rd lock plates separately, so they tell you to buy 50rd mags.
1). The worst aspect of the PS90 has to be the ridiculous excuse for a sight. The reflex is a Ring Sights MC-10-80. You need to shoot it in broad daylight with dark paper targets at the range in order for the daytime reticule to work properly. At night, it has to be PITCH DARK in order for you to clearly see the tritium glow "t" reticule. The Secret Service DID adopt the P90 but they are using a special "USG" sight, which has an aluminum housing and 100% full time black reticule. the standard PS90/P90 sight is cheap plastic (like a Made in China optic) with a white day reticule and tritium orange night reticule. The USG sight is $700 USD.
FN is going to sell a 1913 rail that replaces the MC-10-80 sometime in the future and it will run about $150 USD, but had been sold to LE for a few years already at almost half the price.
Some will say, "But FN has used the MC-10-80 for 15 years..." well look at how many agencies have adopted the FN with the MC-10-80... practically none or they don't like it. Even Houston PD that adopted the P90 complained about the MC-10-80 and preferred the P90 TR (Triple Rail).
Not only that but if you rush into a dark room from the bright outdoors and expect to see the tritium "T", good luck.... No one in the know would trust their lives or the lives of their team members with the standard MC-10-80.
2). Mag changes: quite slow and cumbersome. The mag release pad requires 8 lb-force to depress. It's not much but there's not a whole lot of surface area to push, and you need to use your index + thumb to depress the pad, and your middle + ring finger to lift the mag. Then you must grab the mag and pull it out. There is a lot of fine motor control involved, but with training it will be acceptable.
3). Trigger pull isn't bad, it has a short pull length and requires 7.5-8 lb to break. Think of a regular single-stage AR-15 trigger that you get with a LPK.
4). The charging handle is kind of small but easy to use, I like how its on both sides so you can charge the PS90 with the stock shouldered.
5). The 30rd mags are 50rd mags but with a different magazine lock bottom. There are some people working on custom made 50rd lock bottoms that will convert your 30 into a 50rd mag. You can buy P90 mags and they will work on the PS90.
6). For range use, the PS90 is fine out of the box. If you want to use it for a home defense carbine, I would recommend ditching the MC-10-80 asap and put on a rail for your favorite aftermarket optic. Everything else can be corrected through training (mag changes, safety, trigger, overall ergonomics).
7). Ammo prices are fairly high, right now the average price is about 40-50 cents a cartridge. You can try to find them at 35 cents a cartridge though. LE hasn't been impressed by SS190 with its sub-12" penetration depth in 10% gel (heck, even FN posted the photo on their own webpage). RCMP found the same thing. SS195 and SS196/SS197 hasn't been formally tested and it would be interesting to see how they fare.
This is my no-BS assessment. There are a few others here on ARFCOM that will disagree but they do not have frequent access to a PS90 or P90. Take what they say with a grain of salt.