We needed load balancers and at the time, the choices were ZXTM, ServerIron (which I'd used subsequently, and am very grateful we didn't pick) or F5. We'd had a truly terrible experience with an F5 reseller so we decided to meet with the Zeus guys. One really attractive option at the time was that they would provide us a solaris build of their software (we were a Solaris 10 shop), and we could run it on our own hardware, which was much better provisioned than the comparable F5 hardware at the time.
All of that aside, the motivation was that by using a product, instead of rolling our own solution, would be more maintainable, and we'd have a support contract. Compared with the mod_rewrite mess we had, ZXTM was a huge improvement. Plus we got stats, graphs, IP failover that worked flawlessly, active/active configuration sync, and a webui that made it so people who weren't intimately familiar with the config syntax of a variety of disparate services could perform fairly complex tasks (like A-B builds).
I mean, we also paid for Cisco ASA 5550s as a firewall/VPN solution for many of the same reasons. It all came down to what we wanted to spend our time engineering and what we could get management to spend money on. :)
