I have used a few different PHP based CMS systems and they all seem to use the tinyMCE editor. I have found this editor to be far from wyswyg and almost always requires me to go in and fix things in the html window. This is not a problem for me as I would rather do that anyways, but my clients who I build websites for are really really not able to do anything in the html window. I also always have to go in and edit the tinyMCE config files and such to get it to let me put inline styles and such. Our design firm is extremely picky about how things look and so I have to make sure that things in the editor output exactly right.
I have looked at other editors and found one that might be promising. I however am not experienced enough to implement it on a system as complex as Silver Stripe. I was hoping that maybe someone else might look at this and like it enough to give it a try.
This is a jquery wysisym (what you see is what you mean) editor.
you could have a look at simplehtmleditorfield, which implements jwysiwyg, a light-weight spin-off of wysiwym. it comes as part of a great module by 'unclecheese', dataobjectmanager.
unfortunately it's still somewhat buggy and doesn't seem to get the love it would need from it's creators. also, it can't be seen as a replacement for tinymce, which is heavily baked into the core, but it can be used in popups, which is it's main destination.