I'm in absolutely no rush to upgrade even though I have a lifetime license. I felt that outpost 2008 was too rushed and many had issues in the early install. I would say it was rushed out the door in great haste as it wasn't even Vista compatible, not to mention there was no work (or so it appeared) on OP 4.0 to get that Vista compatible. I don't use Vista so I couldn't have cared less but may users weren't very happy with Agnitum.
Then there is the insistence by OP 2008 that you disable one of the avast drivers, namely the one that supports the network shield even though it isn't a firewall and shouldn't be an issue. This issue is soon (I hope) to be resolved by OP where will no longer be a need to disable the avast driver, then I may update.
As to is it OK to use an old firewall, it depends on why it is old, if the later upgrade are for security reasons then I would say that you should upgrade otherwise you are potentially vulnerable. But in many cases these upgrades are adding new features that essentially don't change what your 'firewall' does (bells and whistles) then you could easily stay with an older version.
It is hard to compare firewalls as the only real test is anti-leak, what can escape from your system and all three you mention do well in tests, well above the rest and the differences are small even if classed as 4th in an arbitrary test, they are still well above the competition gaining 'Excellent' grades. One difference is you have to pay for OP 2008 where you can get free versions of comodo and online armor