First off I noticed that there is rarely any type of lag and seems very good for an online spreadsheets application. You can download your spreadsheets in .xls or .csv or get the html of your spreadsheet.
I messed around with my spreadsheet for quite some time and I must say that the 'share' feature could be really useful. You can allow people to edit your spreadsheet or view your spreadsheet. All you have to do is enter their email address and your friends can get their spreadsheet on. Also you can chat with them via the spreadsheets window which is a necessity.
I tried inviting one of my other email addresses that was a gmail account. I was able to access it and see that myself (my 1st email) and myself (my 2nd email) were viewing it at the same time. With my 2nd email I had not signed up for the beta but was able to access the spreadsheet. Than, I signed out and tried logging into Google Spreadsheets with my non-beta account and it worked! Seems like you can invite people to Google Spreadsheets indirectly. So if anyone misses out on the beta than just post your email in the comments and I will send you an "invite". If you are uncomfortable putting your email than just email me (look in the sidebar and my email is there).
How is this going to be useful in the business world? Why thank you for asking. Example scenario: Say I'm a business exec and I've just made a spreadsheet but I need my boss to check it out and make sure it is ok. I made the spreadsheet in Excel so I just import it into Google Spreadsheets. I invite my boss via email and he logs in and gets to see the spreadsheet. Then, we chat it up and make some edits and when all is said and done we cut out a lot of hassle and the constant sending back and forth of the document.
I believe Office 2007 will have a feature similar to this called Live Meeting; i think. I'm not sure so don't quote me. However, I think it will cost some money which I definitely would not pay for.
One thing that somewhat bugged me is that there isn't as much integration as I would have hoped. Really, you can integrate this with all that much but I found something. In Google Analytics there is a save as .csv file. I did and it downloaded to my computer. I wish it would have given me the option to open it in Google Spreadsheets instead. I gave some feedback to Google and hopefully this will be implemented soon.
Personally, the only reason I would use this is for the real-time sharing feature because I've already forked over plenty of money for Microsoft Office for Mac and trust I'm going to use it if I've got it. Spreadsheets seems to be secure enough but I don't know if business execs fear having corporate info placed on the web (although anyone good enough could just look at emails and get the documents from there).
Overall, I've got to give Google Spreadsheets a 5/5 because it does everything you want it to, works great, feels like an application instead of being online, and it enables real-time sharing.