The iPlanet app server (or Sun ONE App Server or whatever they're going to finally call it) in the 6.0 incarnation has a lot of bugs. 6.5 just came out and supposedly is fixing a lot of stuff, but it's still the same codebase inherited from Kiva and Netscape. As it stands, iPlanet is not a bad server, but it's not a great one, either.
The newest version (7.0?) could potentially be good, but I would wait for 7.1 or 7.0 SP1 or whatever the first maintenance release is before even trying it since it will really be a 1.0 product and not be mature. I'm not convinced yet that Sun can write a truly good commercial piece of software on it's own and have it be good on the first go-round.
Java has been a success story, but Sun didn't intend it to be at first. Solaris is a good operating system. But most of the other software I'm familiar with, such as Forte for Java, StarOffice, and iPlanet server software have been written by others and bought or borrowed by Sun.
Anyway, if you go with iPlanet you'll be ok, but go in with your eyes wide open and make sure you get a maintenance contract to get free upgrades.
Supposedly Oracle 9iAS is good, but I have my doubts about Oracle as well. They make a good core database but my experience with anything outside of that has been decidedly mixed. Weblogic and Websphere have a lot of market share because they're good solid products, and the full weight of the companies are behind them 100% as core products. Macromedia JRun is worth looking at. The new ColdFusion Neo is actually going to be implemented on top of JRun in
J2EE (Coldfusion tags actually parsed by a JSP-like templating engine), so that's worth a look-see since you could use CF and J2EE at the same time in the same app.
I would also look at HP-AS app server - free! You pay for a deluxe version with more features and support, but the free version is full J2EE compliant. Another to keep an eye on is Pramati.
Just to finish answering your question, kkyim, I use iPlanet every day and it works. I don't know that it really has any distinguishing characteristics besides being owned by Sun. It is quite expensive, though. Don't leave it off your evaluation list, but it's not likely going to be a front-runner to win the election.