I had a chance to test it with the 233 and 761 by monitoring the buses passing or stopping at Van Nuys and Sylvan; and I also checked the arrival times of the 761 bus I was on and it was quite accurate, though I did hear a bus (maybe 761) roll through Van Nuys/Sylvan that was not tracked on NextBus (NexTrip, or whatever they will call it) - I'm assuming that bus was not logged in or logged in incorrectly.
I had an hour to kill recently so I monitored the buses passing through the Larchmont/3rd Street intersection against the NextBus app on my iPhone. I was impressed with how accurate it was. I think the weak link is in how often my phone was updating as opposed to any tracking issues with the buses. Being an app, it doesn't allow more closely spaced updates of the data as far as i could tell. I exited the app and re-entered to get updated data, but that was a pain. Overall, it was pretty impressive.
Metro Westside/Central Service Council had a presentation update on the Nextrip program. Here's something to look forward to (summarized from the last page of the handout): Go Metro App - May 2012; Nextrip rail - Early 2012; Testing low power (solar) displays for on-street signage; Real-Time Google Transit – Preliminary discussions to have Metro as a test site. Monthly usage jumped from 102,207 in March 2011 to 1,772,141 in March 2012. I myself have been using the Android-based L.A. Metro Alerts and times listed are accurate to within ± 2 minutes. It'll be interesting to see the rail aspect of Nextrip, along with Google Transit integration.