I use CallWithUs for my outgoing calls and IPKall for my DID. IPKall connects directly to my ATA (SPA3102). People reach me via my GV number which forwards to my IPKall number.
I've never had my incoming calls setup any other way than direct to my ATA, so I can't say if whether there is a noticable difference vs. forwarding IPKall through another number. I can tell you that latency seems very good to me--at least as good as when I was using HyperDog (which I was perfectly satisfied with most of the time) for incoming and outgoing. I do get pretty bad lag when talking to my mom who uses Comcast Digital Voice, but that was a problem when I was with HyperDog and to some degree when I was still using POTS.
Setting it up is easy enough that there's no good reason not to give it a shot. Just login to your IPKall account and enter your ATA's IP (or if you have a DNS resolvable address you can use that) in the "SIP Proxy" field and set the "SIP Phone Number" field to your CallWithUs number. On the ATA (I'm assuming the SPA-2102R has these options) set it to answer without registering. If you already have it set up to register with CallWithUs then that should do it.
You don't have to register with CallWithUs, btw if you'll only use it for outgoing. I have my CallWithUs SIP credentials entered and have register set to no (and "Make/Ans Call Without Reg" fields set to yes).
In my experience, IPKall directly to my softphone is 7 hops while Gizmo5 is 12 hops and the first 4 hops is inside the ATT network so really its 3 hops from IPKall vs 8 from Gizmo5. That's a significant improvement going with the direct route. Of course your network path may be different. Also call quality will not be affected by Gizmo5 since it will not be touching their server.
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