I'm probably just not being clear, you have it correct.
You can set up 10 pop email addresses, the BWC checks these approximately every 15 minutes. It then downloads these to the bb mail server from where it is pushed to your device. You have an email address for this account like
. You can forward mail to this account or email to it directly for immediate push to the bb.
The bb doesn't actually sync with the 10 pop accounts. It just copies the mail from those accounts to its email box. There is a utility available so you can set the reply to address for each message, otherwise when you respond from your bb to an email it will set the reply to address to a default that you specify. The difference with PocketPC and bb is that your PocketPC actually logs into the pop server to receive and transmit. Whatever changes you make with the PocketPC are there when you log on with a desktop client.
If you have a MS Office, a pdf or other supported file type it can be downloaded and viewed only if it was an eamil attachement. That is, you can't copy files from your desktop to your device and view them. I'm not sure if you can copy them and sent them as an attachment.
Working with an excel spreadsheet is next to impossible, but word and pdfs are ok. It is the server that actually converts the Office/pdf files to a viewable format, not the device. That's what I meant, not a big deal if you only plan to look at email attachments and not copy from your desktop.
You plan to use IM, make sure the program you use doesn't send and receive via SMS since SMS isn't usually part of your data plan.
The blackberry has it advantages, it is just very different than the PocketPC. I'll be giving up my PocketPC, not happy about it but I don't want to carry two devices. Checkout PocketDay and berry411 I find both very useful.