Hello zcalin,
You're right, Finland is often divided into three areas where Swedish, länsimurteet (western dialects) and itämurteet (eastern dialects) are spoken. Those maps forget about Saami in northern Finland. There's a number of differences between the western and eastern dialects, I'll make you a list tomorrow.
When it comes to Savo, even though it's usually considered as being an "itämurre", it is definitely a language of its own. Finns that live outside the Savo region have a hard time trying to understand it. Quite a number of books and comic strips have been translated into Savo.
I would say that standard Finnish only exits as a written language, the official "kirjakieli" that is used in newspapers and in TV news for example. Otherwise, wherever you are in the country, there are quite striking regional differences. Each region has its own dialect. For example I spent over two years in Tampere, so I understand the Tampere dialect very well. Each Thursday, part of the Tampere newspaper, Aamulehti, is written in the Tampere dialect. There are even news in the Tampereen murre on the radio.

Now that I live in Helsinki, I'm getting used to the local dialect, stadin slangi.
I can also tell you that the dialect Finns find most horrific is the Turku dialect. For me it's ok, the ones I find really hard are the Oulu and Lapland dialects. My boyfriend is from Oulu and his grandfather from Rovaniemi, so when we go there, it's horrible for me just because of the way people speak in casual situations.
A series of small "Katekismus" have been written in the different Finnish dialects. It's more like for fun than a serious project, but it's a good way to compare the different dialects.
