I have a UR44 as my backup unit (for my Octa-Capture which has a bad headphone jack). 
Setting it up in Sonar with a buffer size of 128 samples, I'm showing almost double the latency I get with the Octa-Capture. 
 
Here is what Sonar reports:
Input 6.7 msec 295 samples
Output 7.7 msec 338 samples
Total Round rip 14.4 msec 633 samples.
 
Is this about what it is capable of (reliably)?

I run mine at 64 samples without trouble, but my projects are pretty lightweight. Total round trip is 9.1 ms.

Thanks.  I'll try a lower setting and see if it works OK (but I did get the Octa-Capture fixed - the other thread) so the UR44 is back to being the backup unit.

Many (perhaps most) interface drivers have a hidden safety buffer, so the reported latencies may not be accurate from one interface to another.  I don't know if there's an accurate, consistent way of measuring round-trip latency... maybe CEntrance's latency utility?

I get Input 6.4 msec and Output 7.4 msec @ 128. 

Rimshot
I get Input 6.4 msec and Output 7.4 msec @ 128. 

Almost the same as firebird.  I'm guessing one of you is at 48khz (and the other at 44khz).

I'm at 44

I'm at 24/48

I'm at 48

As long as the driver reports the proper latency (including the safety-buffer), Sonar should report it correctly.
The UR44 has higher RTL than the Roland "Capture" series (which go down to a 48-sample ASIO buffer size).
 
I believe the Capture series yield 7.4ms round-trip latency at a 48-sample ASIO buffer size 44.1k