I never switched from Netscape, really -
The switch from Mozilla Suite to Seamonkey was made against a cacophony of support for Firefox. Firefox then was like Chrome now - lean, mean, the future, in a word: cool.
People bitched and moaned about how the Mozilla Suite (and, by extension, Seamonkey) was burdened by bundling its mail, news, chat, and html edit programs together; people wanted a lean-and-mean browser.
The tables are turned now, though. By avoiding all the pointless cool chrome (to use an expression), Seamonkey has managed to stay feeling light and purposeful.
Add to that the fact that
- the UI is stable
- the version numbers are sane (and the release schedule is sane, unlike what the current top post on this story says - maybe one minor release per month. very manageable)
- the prefs don't talk down to you
- it has mail and chat attached by default (I like that!)
- it has a single address/search bar
- it uses Gecko, so under-the-hood it's up-to-date
- when you spawn a new tab, the new tab appears at the extreme right, instead of displacing the existing tabs by spawning to the immediate right of your current tab
- the new-tab button is fixed in the extreme left of the tab bar, and doesn't jump around depending on how many tabs you have open atm
There are probably other things I could list. But in general, it _is_ a browser for people who know what they want: a browser that has a perfectly workable UI and does not change based on fashion. And a browser that has a modern HTML engine.
Unless and until the HTML engine becomes stale, I see no reason to change. I like my menu bars, I like spending a few extra horizontal pixels up to have back, forward, reload and stop buttons, I like having an attached mail client. Good design is good design no matter what decade it is.