You also have two ways to go with this , i.e. POP3 connector
Assuming that each user connects into their own mailbox, you can specify user mailboxes in the POP3 connector.
If you get a new user then you have to setup a new mailbox with your isp and a new mailbox in the POP3 connector to map to their Exchange mailbox.
Pros... Easier to setup
Cons ... An effort to maintain if you have a high turnover of staff
OR
you can have a catch all account with your isp and then setup a global mailbox on the POP3 connector.
It will then filter by recipient and sort to the corresponding exchange mailbox.
Pros... Easier to maintain, no more adding accounts with your ISP or to the POP3 connector, just their exchange mailboxes.
Cons... Migration needs to be meticulous.
Mmmmmm choices choices...
you're also limited to picking up mail every 15 minutes but there's a reg hack that reduces this.
Linky
You should long term be looking to setup MX records to point to your mail server so that you receive mail directly and instantly through SMTP.
Disadvantage of POP3 connector is that it bypasses the Intelligent message filter so bang goes your free spam filtering in Exchange.
It's sufficient though if you find your ISP to filter effectively.