You can customize the appearance of quite easily Sharepoint and Sharepoint-like things you might want to do (usually around lists, document libraries and workflows) is a very good tool.
There are problems about personalization in some respects - the moment they begin to do things other than the "Sharepoint Road" (which is often necessary to fairly common things you might want to do) can be found running in circles, jump through many hoops and generally have to go to C, D, H, J, Q and Z before you can get from A to B. Fortunately, a fairly mature product as it is now, you can often find blogs covering the issues that are difficult or commercially available web parts that can be licensed instead of writing your own. For some standard things (Sharepoint wikis are attrocious, for example) may be better to use a licensed component of the construction site in regardless.
Sharepoint can also behave quite inexplicable, if you're thinking about using different types of user authentication for intranet and extranet users you want to see up close how best to implement it. Not impossible, by any means, but difficult.
In general I think SharePoint is a very good platform and it is worth considering seriously. It is almost certainly better than starting from scratch and probably good enough for what you need, but definitely should do some evaluation and prototyping before committing themselves.
Note that to get the best of Sharepoint 2010 will all have installed Office 2010 as well.
