Pros
Software is used by major financial institutions and can lead to lucrative contracting opportunities once you've left the company. Some Dev teams are now embracing agile methods (scrum) instead of the 'back of a beer mat' approach WSS has traditionally used. The parent company has some interesting software.
Cons
The current software is getting a bit old and though new software is being developed how well this will replace the current software is yet to be seen. Pay rises are low and bonuses have been scaled down. Managers can't even hire for junior positions without waiting (sometimes months) for the personal approval of the ION CEO this has lead to losing good candidates. There is a fairly flat management structure and chances of career progression are slim. Even if you do progress then your hands will be tied, decisions are made from the very top. You will be made to sign a non-compete, they do try and enforce this, sometimes successfully - leaving the company can prove to be tricky. The group as a whole is very fragmented, it is a collection of lots of small software companies. There is very little transparency. Some of the same issues now faced by WSS staff can also be seen in the reviews written on here under ION trading (the parent company). Most of this seems to stem from the rather mercurial ION CEO.