I'm afraid we're still hanging 
I checked the manga against Wikipedia's episode list for the anime and it looks as though you'll be fine to start at volume 12, which gives you almost twice as much spiralling drama to enjoy again before running out if you decide to have more. Though you're probably in a better position to stop now than if you keep going. I love it though despite the abrupt unintentional ending we have at the moment.
The manga also contains standalone bonus chapters occasionally and it should be noted that volume 9 had one of these, focusing on Naoki from Trapnest. It doesn't look as though his story is included in the anime at all, so it might be worth checking that out too if you decide to go ahead.
I'd have said more of the anime was a certainty a couple of years ago, but with the manga artist's situation the hype surrounding the series seems to have faded quite a bit. It's one of the reasons I didn't buy it on DVD (that and the song replacement, and the fact that I hate and distrust Viz's anime side).
I believe the song was removed due to a plagiarism issue ( http://www.youtube.com/watch?v=X3vF9fdYYWQ ) and Viz were afraid to get involved. Or so internet conspiracy theories go. The Olivia song is a million times better :/
I have the physical version of the iTunes album which comes highly recommended (the physical version has the missing song, too). The first press with the sexy DVD is long gone but if Japanese music prices don't make you cry the regular edition should still be around. Recorded Butterflies is gorgeous too.
This runs into my next comment
I agree that they are similar as well, but I'll add that I would have described NANA as a more character-centric story whereas Solanin is all about the mood during a very specific time in its characters' lives. I didn't find myself liking the characters in Solanin at all; part of this was that it was so short there was barely any time to get to know them, and part of it was that the underlying themes of Solanin didn't really resonate with me. For all that the lead was as cute as a button, I found her personality annoying and difficult to sympathise with.
That's not to say I didn't like Solanin - I did - but it set out to achieve something quite different to NANA and with its narrower focus I wasn't sucked in as much. I imagine that if I'd identified a little better with the leads it would have worked a lot better in my case.
This leads into NANA in that my own personality is definitely much closer to Nana's than Hachi's, for better and worse, and there's a huge cast of characters that I ended up becoming quite close to. I love most of the NANA characters dearly, even the ones I also want to strangle at times for being stupid. It's impressive that Yazawa made me care about so many messed up characters so much. I could never imagine being anything like Hachi or going through the things she does, but well-written fiction such as NANA helps me understand and make a connection to the kind of person she is from a distance.
Perhaps I just prefer series with large casts in general as there's a higher chance of my wonky personality type being included
R (Ramble over! Though I don't know why I'm even talking to someone who hates Blue Seed *huff*)
