Well, every day is probably a bit too frequent to check for updates. But it is worth checking (remembering to update the slapt-get data before you do so), since the packages in testing will (gradually, given the amount of time I have at the moment) be migrated to the normal repos.
One area which is not covered as well as it should be is security related updates. For instance, the most recent versions of Firefox, Seamonkey and Wireshark are all secuity-related updates. I normally try to indicate when that is the case, but, again, its a matter of how much time I have, so it doesn't always happen.
For that type of update, depending on your situation, it may be worth looking at updating before the packages have migrated to the normal repos.