In terms of carebearing I recently tried the 'guristas provincial HQ' plex a few times . It's one that I've long eschewed because the escalation end guy takes 2500 dps to kill, which I didn't have but can now manage in a pinch. What I found was that because of the siege railgun sentries in the plex, a standard plexing ishtar fit does not work well -- although it has good resists, the tank is paper thin and can break if the sentries don't miss often enough. One option would be using faction reppers and/or having the accompanying battleship use repair drones or RR; another would be to upgrade the tank on my scout/scanner/tank tengu and use that.
A few months ago I posted this tengu fit which I was using as a plexing utility ship, being able to fill the roles of scout, scanner and tank. I modified it to a more focused fit, making it a weaker scout (MWD removed, align time increased) while dramatically increasing the tank against guristas and keeping with my tradition of not using faction or deadspace mods.
There are three plex with special cases that the fit needs to work in:
First, the final room of pith's penal complex. Although the old fit *could* tank this I was forced to travel away from the NPCs to keep damage down, which was annoying. The new fit tanks the final room like the NPCs aren't even there.
What missiles blotting out the sun?
The second is guristas provincial HQ, which gave my ishtar problems as noted above. The new tengu fit has no trouble at all there.
Finally, guristas military complex. The smartbomb in the second room does 6000 EM damage, and the tengu has 4400 shield and 0% base EM resist. I use a shield EM resist rig, which gives 30% EM resist and prevents the ship from taking armor damage when the smartbomb goes off. Note that replacing a kinetic hardener with an invulnerability field would give 30% EM resist without increasing the signature radius, but the change in signature radius from the rig is only about 5%, whereas the difference in kinetic resist from using an invuln (90% instead of 93%) increases incoming kinetic damage by 42%, which is sort of a big deal.
Also note that I changed HAMs to heavy missiles. The slight reduction in damage is more than made up for by the increased utility granted by the greater range of heavy missiles.
EDIT 2010-12-14: Two medium boosters have the same boost amount and cap requirement as a single large booster, so use that instead. I put an invuln in the spare mid spot, which means that the EM resist rig is no longer necessary.
Now all I need is to find a fit (non-scanner/scout) that can tank the maze without RR support. It's possible to get ridiculous guristas tanks out of the tengu but there's a question as to how much buffer and EM resist it needs.