For watching any recorded shows, it is very rare for any buffering to make the show unwatchable or for it to be a significant issue. I suppose if you were trying to watch the latest Game of Thrones episode within an hour or two of broadcast, you might have some issue due to volume of traffic. If you decide you are going to binge watch a season of something like you would on Netflix? No - no issues at all.
If you are watching a live sports stream via Mashup or trying to watch a live stream of another cable channel, then yes, there can be lag. Depending on the channel, there can be a lot of it, too. (Trying to watch HBO via live stream on a Sunday night is just not going to happen). For most casual viewing, this is not usually a problem though.
For watching a live Sports stream other than through a subscription service, its always hit or miss. Servers go up - they go down; sometimes in the middle of the game. Games come in crystal clear, or they are so poor in res you are wondering what sport you are watching. The more popular the sport and more critical the game -- the worse the lag and picture quality. The more you care - the worse it gets.
For casual viewing that may be fine for some people. If you are a sports fan, you should subscribe to one of the pay streaming services mentioned on page 1 of this thread to avoid those issues.
A premium subscription sports stream (MLB.TV, Hockeystreams, etc. ) that is merely being watched through XBMC is generally lag free at all times.