There are a lot of variables here, but in general, the only way a subwoofer can really improve a system is by easing the load on the speaker's bass drivers, allowing them to concentrate on a smaller frequency bandwidth, which should allow them to sound cleaner. Of course, this depends on the capabilities of the subwoofer used. Any other 'improvement' is a result of how we perceive higher frequencies when we boost lower frequencies. The boosting of lower frequencies isn't necessarily an improvement in itself.
For movies, any system will benefit as there are low frequencies mixed into some film soundtracks that can reach below 10Hz, although to take full advantage of this sort of frequency response, you'd need a surround processor that can decode HD audio formats which can then feed a subwoofer a direct line level signal. Music is a different matter though. Personally, I'd say that it depends on the music you listen to. Most music based around real instruments tends to be fine on most good quality speakers, but there's a lot of electronic music out there that produce bass well below frequencies that most speakers can handle.
A very high quality, very capable subwoofer though will cost in excess of £1,500/2,000 (exactly how much depends on how low you want it to go, and how loud you want it to go too), and for some, this could run to £3/4k, so you then have to weigh up the pros and cons of using your existing speakers (same quality) with a sub (more and deeper bass) against a better quality pair of speakers (higher quality across it's entire frequency range). Many would probably plump for the latter.
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