I haven't had any problems, but if the problem affects some Web sites and not others, the first place I'd point my finger is a problem with your ISP's name servers.
There's a quick way to test that. Open the Network pref pane on one of the affected Macs. In the Name Servers filed of the TCP/IP settings, put
8.8.8.8
8.8.4.4
These are the Google Public DNS settings. If you do this and the problem vanishes on that Mac, you will have definitively identified the problem as your ISP's name servers.
