Couple of variables
- How well do users know standard notation
- How many units of measure do you have to deal with
If users mostly know and stick to S.I. and other well defined units, then it will be enough (efficient, non-distracting and clear) to just confirm an entry with full text version after completion i.e. [250 m] entered becomes [250 meters]. For assistance just link to a unit reference.
If the full gamut of units (i.e. say all options for acceleration, light intensity, ... ) is available to the user, then even a filtered list is likely to be noisy (showing rare units, users choice not visible early, large scale of updates). See the number of hits found in this unit dictionary for "milli" and other common units. If do decide on doing an auto-complete for the full gamut of units, make sure the most commonly used units get highr precedent in the filtered list.
If you can limit number of units in play i.e. only need to include common Length and Area measures and the users are very in-accurate in their unit abbreviations then auto-complete should be useful.
If have to have full gamut of units and their are novice users, then consider establishing nature of the measurement. Heat, Speed, Length, Acceleration, etc and providing a full browse option.