When I was first learning Japanese, there was a magazine called "Mangajin" being published. Basically it used manga to give lessons on Japanese grammar and culture. There is some info on it at
Wasabi Brothers Mangajin Magazine, Online Japanese Catalog and Store but it looks like it is long dead now... anyway...
As Paul said, you can't learn Japanese from manga alone, because generally it isn't polite enough. Even something like Kacho Shima Kosaku probably has too much casual Japanese. If you don't learn polite Japanese, and use the casual style in formal situations, it's not so much that you "offend" people as that you sound like a complete hick/moro n.
Personally I started using manga when I decided I wanted to learn kanji seriously, which was about a year after I started studying Japanese. You can get to an intermediate level in Japanese without kanji, but eventually it just becomes too hard to learn new vocabulary because you can't "store" everything in your brain using only phonetic cues. There are just too many homonyms, and you aren't linking the syllables to anything - the kanji provide a kind of an "anchor" to link the sounds to, when you are filing them away in your brain.
It would be very hard to learn from manga if you didn't already have a decent level of conversational Japanese, because you have to make a lot of deductive "jumps" to work out what people are saying. Like I will just pick up a manga that is sitting on the sofa right now, flick through it randomly (it's Ranma 1/2). ムース says "えーかげんにせんかーい！！" (ee kagen ni sen kaai!!) which should be "ii kagen ni shinai ka!!" in standard Japanese ("stop screwing around!!") You kind of have to know the standard version before you will understand the slang version.
Like okaeri_man said, Doraemon is a good manga to start off with. It's clever and has lots of Japanese cultural information about family relations, children's hobbies (at least until the 80's or so), holidays, fairy stories, and so on. It's a classic that probably almost all Japanese have read. I think the early books don't have furigana though.
My favorite is Crayon Shinchan. The anime sucks but the manga is great. You can learn EVERYTHING you need to know about Tokyo-style conversational Japanese from Crayon Shinchan. It teaches you how to make puns and plays on words in Japanese. I couldn't tell you how many times I have blown Japanese people's minds by recycling jokes from Crayon Shinchan. They stare at you with their mouths open.
Recently I got Love Hina which is also pretty good, especially if you are thinking about getting a scholarship for a Japanese university. The characters speak a standardized version of the way young people speak to each other, without too much slang. It'd be good for internalizing the right level of politeness for speaking with university-age Japanese, and it has plenty of examples for both men and women. However it's a total mystery why all the women like Keitaro so much, especially as it progresses... a bit of a fantasy on the part of the author if you ask me, who is really really nerdy looking.
