The KDE4-version of Kaffeine is stable, but feature-wise under heavy development, many funtions are still lacking.
I recommend using SMPlayer, which features extensive options for configuring separate subtitles such as .srt and .sub. - it is even able to fetch multilingual subtitles from opensubtitles.org, which is really cool.
One more annotation on Kaffeine which I forgot to write down; it actually is able to include subtitles (see bottom of the Kaffeine-window, there’s a respective dropdown-menu), but it is not possible to point Kaffeine to a subtitle which is outside the medias folder. Also I do not see a way to configure the font, colour, size, delay, shortcuts etc.