This seems to be one of the few subjects that hasn't yet been covered in this forum (unless I've missed the post and it has), so I thought I'd write something about it.
Systems of measurement might not sound like the most interesting of things, but they can be used to good effect in a role play campaign. There are three main ways I can think of: 1) Adding colour; 2) Culture shock and 3) Archaic systems.
1) Adding colour.
Of course, there is nothing wrong with doing a campaign using standard imperial (or metric) units. However, inventing your own system can be used to add a bit of colour to the campaign and, unlike some other things you can do (such as inventing your own language), is incredibly easy. If you don't want to complicate things too much you can quite easily make the new measures reasonably similar to the others (after all, a foot was based on the length of a foot, etc. so it's reasonably to assume that other cultures might have similar measurements). Even if you want keep some of the basic measurements such as feet and inches, why not change the length of a mile, or always work in leagues? There are also plenty of ancient imperial measurements which you can resurrect, such as chains, furlongs, roods, etc. However, its once you've decided on a measurement system for your culture that things can really get interesting. Points 2 and 3 are equally useful whatever system of measurement you're using as your basic one.
2) Culture shock.
Suppose the PCs travel to a different continent. What makes you think that that continent will use the same system of measurement. After all, Britain uses a different system to the rest of Europe, the differences could be even greater in a world with much less rapid travel and communication. By using a different system of measurement, you can really bring it home to the PCs that they are not just in another port town, but in a totally different part of the world (of course, there are lots of other things you can do in conjunction with the measurement thing). This is something that should hit them every time they ask for directions or walk in to a tavern. Whilst you can continue describing the scenery in normal measurements, if every NPC they speak to uses the new one, they'll initially not know what anyone's talking about. "So, how long is 1 Kheldam? Yes, I know its 7 Onacs, but how long is one of those?" After a while, of course, they'll start getting used to it, but they will keep having to think about it (and to translate "normal" measurements in to it if they speak to anyone), which should serve as a constant reminder that they're "not at home".
A potentially much more interesting thing you can do is to keep the system the same but just change how big one of the units is (whilst keeping the same name). Again, there is precedent for this: in the US, 1 pint = 16fl oz but in Britain 1 pint = 20fl oz (this also causes the gallons to be different sizes). More significantly, in Norway a mile is 6 times longer than a "conventional" mile (i.e. the one used in Britain and America that is about 1.6km)
Imagine springing this on your PCs. They ask directions to a nearby town and get told it is 10 miles away.
Easy, they think, "We can make that in a day. So, they set off and, 18 miles later, night is falling and they're forced to make camp in the middle of the wilderness. As well as setting up a situation for any kind of encounter you might wish, it will also totally baffle them. They will probably start wondering why the person they met lied to them. If they read the distance off a map, they'll wonder what is going on. When, on the eve of the next day, they still haven't reached it they may decide to return - potentially costing valuable time and, depending on the nature of your group, may react in all kinds of ways to the NPC who "lied" to them. Of course, no-one else will understand what they're getting worked up about, particularly as they (probably) won't get what's wrong either. Incidentally, this happened in reverse to a Norwegian friend of mine. Fortunately, as she had come to England, she had a pleasant surprise (she was told somewhere was 1/3 of a mile and decided to walk it, thinking it was about 3km when actually of course it was 500m).
3) Archaic measurement systems
In the past, society probably didn't use the same system of measurement as now. Furthermore, since these old systems are now dead, there was probably something wrong with them (in that maybe they weren't so easy to use: think of the way metric is gradually superseding imperial). It's quite nice to give your PCs a set of directions to the treasure that they don't understand because the units are archaic. This can spark interesting side-quests to find someone who can translate it. Another thing is to let them find a map of the dungeon they're in which marks all the traps, but in an old measurement system. Of course, they'll figure it out eventually (and then the map will be very useful), but until then things will be fun. For some reason, having a map you can't understand which tells you where the traps are seems to be much more infuriating than just having to look for the traps without a map.
If you're GMing a group which enjoys puzzles and riddles (which I am at the moment), you can have a lot of fun with the fact that old measurement systems might have been rather illogical. For example, we use logarithmic scales to measure sound and earthquakes: maybe an old system used a log scale for distance. Perhaps the most bizarre one I've created was the Karn system of measurement. In this, the length measurement had the zero point at 62 and went up in 4 increments (i.e. 54' was actually -2 Ka). This might sound crazy but we all accept it in temperature without a second thought. Of course, strange things happen when you start adding things up. Anyway, there were only two clues I left to figure this out (figuring it out was fairly necessary to their having much chance in the abandoned ruins they were in): the first was a broken ruler that showed the difference between 2Ka and 3Ka, and 1Ka and 2Ka was 4; the second was a scrawled statement saying 2+3=20. They did manage to get it in the end.