Some of the downsides / things to be aware of:
*If possible, avoid installing any software you may have received with your kit. The vast majority of it is unnecessary bloat. Support for PPPoE, the method by which SBC Y! allows users to log on and have access, is built into Mac OS X and Windows XP, and is easily added with a very small free driver to Windows versions greater than 95 but less than XP.
*Don't expect the special Yahoo features to work reliably or as you might expect. Email and the custom home page work fine. For example, the unlimited Y! Photos space is nice, but the photos aren't directly linkable. The Y! Briefcase space does have a large storage limit, but each file can't be very big. Launchcast Plus only works in an awful combination of Netscape & OS 9/Classic on the Mac (one of my sore spots). It would appear the "SBC Premium Video" service is in the process of becoming Mac compatible, but currently only works on Windows (and that is not official, just based on my own observations).
*I've never gotten SBC's official speed test Java applets to work in any browser, but I use the speed tests at DSLReports.com, and they seem to work fine.
*The TOS used to contain a provision that appeared to indicate that a formerly free Yahoo ID, once merged into a paid DSL account, would never be free again, and instead Yahoo would charge a $9.95/month fee to maintain each ID you wanted to keep. The only way to get an ID back free would have been to cancel it entirely and try to sign up for it again before someone else got it. They seem to have eliminated this provision as of late, but I would still not merge an existing (free) Yahoo ID into the system as a paid DSL account ID. I have intentionally kept my old free Yahoo ID separate from the new paid one. I don't know if they'd add that old idea back to the TOS, and wouldn't want to be caught in that situation.
There are also upsides:
*The newly upgraded email to 2GB is nice (at least on the primary account, I don't use the 10 subaccounts so I don't know about them). Also nice that anything filtered and moved to the bulk mail folder *doesn't* count toward the 2 gig limit.
*SBC doesn't block any ports except 135 (IIRC) and if you do really need it for (some type of Windows network, I think) you can call them and they will unblock it just for your account. They used to not block any, but the Windows virus threat became too great.
*They still offer Usenet (which I think some ISPs have dropped) - server details found in the help system
*They've recently upgraded their phone system to be more "natural lanugage" (seems to be powered by TellMe). Obviously, YMMV as to the usefulness of this, but I prefer it to pressing buttons.
*You can have 1 DSL & 1 Dial-up connection active simultaneously (or 2 Dial-ups) and the TOS allows it.