I've had an IPsec connection to a Watchguard x1000 for a little over a month. I'll be posting some screenshots and a basic howto shortly. (hopefully this week) I will put up the screenshots first, as soon as I can edit out the important stuff.
The short answer is yes, IPsec to watchguard is possible and so, far, seems quite nice.
Pay attention to the "advanced" button when setting up the tunnel on the watchguard side.
Remember; both sides require identical settings for protocols, renegotiating timing, and identifiers.
The default settings do not match between pfsense and watchguard.
This is Monday. I hope to have some images up by Thursday/Friday. (depending how my "real" job goes…)