Westbrook's handles can only be measured by the eye test, but we can at least agree that his handles are at least average, right?
That depends on what subset of players we are comparing him to. If we are comparing him to all NBA players, then, sure, he's well above average as a ball handler if you include centers and power forwards. If we are just talking all NBA guards, then (just off the top of my head) I would say he's probably slightly above average. However, if you are comparing him to the top ten to fifteen point guards in the league, I would say he'd probably be close to the bottom of that group as a ball handler.
Just to put some quantifiable numbers on this:
Among full-time PGs last year (guys who played at least 25 mpg), the average assist & TO rates were:
Assists per 40: 7.4
AR (rate of assists per possession used) : 27.60%
Turnovers: 2.9
TOR: 10.94%
A:T ratio: 2.48
For Westbrook:
Assists: 8.5
AR: 22.73%
Turnovers: 3.8
TOR: 10.30%
A:T : 2.21
So, Westbrooks' turnover rate is average for a PG, but his Assist-TO ratio is below average. This is because his assist rate is well below average. This suggests that he's having fewer passing TOs and a larger portion of his TOs are on the dribble. 82games.com shows that 107 of his 273 TOs were 'ball handling TOs' - 39%. They give him a 'Hands' rating of 26.3.
For comparison, let's look at Rondo:
Assists: 11.8
AR: 39.05%!!!!!!
TO: 4.2
TOR: 13.79%
A:T: 2.83
Rondo's turnover rate was higher, but that was clearly driven by his much, much higher passing rates. When you consider how poorly the team around Rondo shot while he was on the court this last year, those assist numbers don't probably fully reflect how much Rondo passes the ball. Despite that, his A:T ratio was well above average for a PG. 82Games.com indicates only 33 of Rondo's 148 TOs were 'ball handling' - just 22%. Nevertheless, because he had a higher share of passing TOs, he ends up with the essentially the same 'Hands' rating as Westbrook: 26.4.
The net of all this is that Westbrook is somewhat below average as a ball handler and loses his handle more than the average full-time PG. Because he doesn't dish out as many passes as the average PG, his overall TO rates are not unusual, though.