The negative reviews are only in comparison to S8, which is kind of true in that the changes are mostly just under the hood. S9 is like a "S" release for Apple, compared to S8. But comparing to S7, everyone I have read says that it's a worthwhile upgrade.
Personally, coming from S8+, I feel that my S9+ is an incremental upgrade - screen is slightly brighter, cameras are better, system is overall faster (time will tell). Battery life is about the same as S8+, so is the overall feel of the phone.
Comparing to S7, you will feel a huge difference in both performance, functionality and form factor. I think you should do it - Get the ultimate bundle with S9+ with Unidays for $876.
https://slickdeals.net/f/11373547-99-ultimate-bundle-unlocked-samsung-galaxy-s9-665-99-or-s9-776-99-with-epp-or-edu-discount