IPsec is an open standard technology so it should work fine. If you already have Cisco IOS routers in place at the remote office(s) look at upgrading to an IPsec IOS. Otherwise I would either drop a small PIX (maybe a 506) http://www.cisco.com/go/pix behind each of those site routers (having the added protection of a great firewall) or maybe a VPN3000 http://www.cisco.com/go/vpn3000 (but that might be overkill for <20 users unless you need remote access to the remote site (i.e. sales people with notebooks dialing in from the Internet cloud).
I have found reference documents for this topic at Cisco "Sample Configuration: IPSec Tunnel - Cisco Router to Checkpoint Firewall 4.1" and at Chekpoint " Check Point VPN-1 and Cisco IOS Gateway to Gateway IKE VPN using Pre-shared secrets".
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...