During my last survey sessions I tried to gather maxspeed information of the streets surveyed. The maxspeed tags are explained and disussed on this page [1]. There are some things which I noticed.
1. Outside towns and villages the default speed limit is 100 km/h. The approach to a crossroads outside often has a speed limit of 60 or 70 which ends after the passing of the crossroads. This has the effect that the speed limit for each of the two directions do not cover the same ways. A possible solution to this would be to define the speed limit for each direction seperately. There is a proposed tag "maxspeed=forward:70" "maxspeed=backward:100" which is currently under discussion here [2]. There is a possible problem in that this proposal depends on the direction of the underlying ways which could be changed later, perhaps by a different and unaware user.
2. Sometimes the speed limit of a given road does not match the actual speed which can be reasonably driven on this road. For example there is a tertiary road outside town which is winding and narrow. It has a speed limit of 70. Parallel to it runs a track with asphalt. It has a speed limit of 100 (outside town) because no one put a speed limit sign on it, and it has no access tag. The track is very narrow, like for one traktor. Of course it is practically impossible to go 100 over this track because of the curves. Should a navigation program route over this track instead of over the tertiary road? Perhaps this should be addressed in the navigation programme. The same could be case with very narrow village streets where there is no speed limit sign (implicit 50 km/h limit), but this is impossible to drive. On other, broader and straighter streets in the village 50 km/h can be achieved. Some professional map providers tag "average speed" in addition to "maxspeed". The problem of course is that there is guesswork involved.

On this page you can see an overlay of maxspeed on the OpenStreetMap tiles to show where maxspeed is tagged and where it is not yet implemented. [3]

[1] osm.wiki/Key:maxspeed
[2] osm.wiki/Proposed_features/Conditions_for_access_tags
[3] http://wince.dentro.info/koord/osm/KosmosMap.htm?zoom=11&lat=6584584.24356&lon=680129.86234&layers=BT