I like Combros - I've had mine for about 4yrs and it's worked well on PCPs, springers and Co2 rifles.
I tested it against a Chrony about a year ago and, they were about 4fps different.
Also, I've only ever heard good things about the service from Robin
If you're only checking your own rifles from time to time, they're hard to beat but, if you're doing a lot of testing on a regular basis, you'd probably want something more comprehensive.