1. Regarding dropbox integration. Yes, it is very simple to import/ export files via dropbox. To import, you just install the dropbox app on theiPad, then open the spreadsheet in it, choose the 'Open in Numbers' option & it will open for you. Exorting is also easy, you just click on the wrench in Numbers, choose "open in another App", select dropbox & choose the folder you want to store it in.
All that being said, I don't care for using Numbers on files that I will be transporting back & forth frequently (like ones that I use in Excel on my Mac & also on my iPad. I feel like the two-step process to download/ upload the output causes too many files, so you download it from dropbox, play around with it on the iPad, then upload it again, now you have potentially 3 copies of that file-2 in dropbox & one in Pages. I find that inconvenient. For files that I work on alot in both places, I use cloudon or the web apps in Skydrive to manipulate. They aren't as nice as Numbers on the iPad, but I can open up & save the exact smae document on both devices.
2. I think that if you go into it thinking that Numbers is 'Excel for iPad', you will be disappointed. I was frustrated with it when I first used it, until I took an afternoon to watch the videos on how to use it & read a bunch of links online. Numbers does the same thing as Excel (for the most part) but it works very differently. I believe it is the best spreadsheet app out there for working on an iPad, but the processes are just different than Excel. Until I took the time to learn how it worked, I was continually frustrated with iPad Numbers. Now I use it all the time when I am doing spreadsheets on the iPad (unless, like I said above, it is one that I work on on both my Mac & iPad.)