The 90mm is sharp, any lens around that focal length should be, they're "easy" to design. The major drawback as I see it is that it's the old tilt-shift mechanics, ie you can't adjust shift and tilt in independent directions like you can with the newer TSE-II lenses. It will depend on your shooting style how much you suffer from that though. They are also not "L" lenses, ie the weather proofing etc is not at the same level as the TSE-II.
Rumours of updated TSE 45 and 90 (the 45 is unlike the 90 not very sharp) has circulated for years but so far not materialized. Now with the high res 5Ds coming the interest in TS-E lenses should increase further so I hope updates will come.
The Digital Picture has some pixel peep test chart shots which you can look at to get some idea of how sharp it is,
http://www.the-digital-picture.com/ , best if you use side-by-side comparison with some lens you already know.