Patton 411X is the favorite SIP PSTN Gateway...by quite a margin...Several things about my experience with the Patton Smartnode series of gateways that I imagine gives it such good ratings:
-Great Tech Support: My experience with Patton technical support has been stellar. They respond in a timely and consistent fashion to support requests. They are willing to dig into tough issues like multiple subnets, faulty pstn lines and whatever. I'll also admit that each time I call them it ends up being something "kinda dumb" that I did--but they pointed it out and got me in the right direction.
-Trouble-Free: We have had very little (I should probably say No trouble) with Patton hardware up to this point. Just set-and-forget.
-Warranty: I have only been involved in one warranty incident in all the units we'ved shipped and put out in production. That one incident was a case where there was no surge supressor and the in place telephone line damaged the Patton unit. Patton replaced TWO units damaged in that way. So it looks like they have a generous warranty too. (don't count on zapping them though! ;-)
On the cons side I do get complaints they are a bear to learn the GUI--a complaint I don't disagree with. Thankfully 3CX makes a config file and for other IP PBX vendors you can usually have a template config file you edit slightly to get going.
Some resources I've made available you might have interest in:
How to Debug a Patton
Kerry's Garrison How to Config a Patton: