Depends. As a trail rig they are fine, you could easily fit 31's on a 1 or 2 inch lift and make it through most places on scenic trails.
For rockcrawling... Not so good, but comparable to other IFS vehicles like Chevys and Toyotas they are comparable. IFS sucks period, doesn't matter the make.
I will start off with the bad points first:
The first downfall is the Tierod adjuster, they are extremely weak and I have broken one every time I have done anything serious. Luckily they are cheap and easy to change. Independent 4x makes some Heavy Duty tie rod assemblies that will fix this problem though.
The 2nd major problem is the front wheel centerline is too close to the firewall to allow for full stuff with 33's without rubbing. A fix to this would be Diff Drop brackets, which you can make yourself for free as long as you have a welder.
The third major problem is the IFS... this is self explanatory, its IFS. IFS just cant compare with the travel of a SAS, so for anything serious you are looking at doing a SAS. This is really a non issue because you are in the same position as any other vehicle with IFS.
The Good:
I wont write out everything good about an Isuzu because IMO they are simply very good trucks, its a shame the 4WD community has not embraced them more than they have.
FULLY BOXED FRAME, Isuzu frams are fully boxed from front to rear. Look around in the Chevy, Ford and Jeep forums about all the work people must go through to fix their Shitty C-Channel frames. Its ALOT of work and it should have been something done from the factory.
You can do a 3-4" suspension lift FOR FREE. The front is torsion beam, and unlike Chevy/Ford, you dont need no stupid torsion keys to lift it. To lift the front all you need is a socket or a wrench of the appropriate size. To lift the rear you have 2 choices, the easiest choice is the longer shackles. Again, unlike Chevy/Ford there is no need to fuck around with some stupid shackle flip brackets, just put on longer shackles are you are good to go. Alternatively (and the preferred method) is to convert the rear to Spring over axle, you can reuse all existing parts like UBolts and Shackle perches, all you need to do is grind cut off the existing shackle perch flip it to the top of the axle and weld it back on. its a cakewalk. For more lift in the front you can also do a diff drop, this is free also as long and you have a welder and basic tools.
The drivetrain is STOUT, the 3.2L V6 makes like 170hp, the MUA5 manual transmission is solid and the Transfer case is gear driven, not chain driven. I have never seen or read about a failure of the Transmission or TCase. The rear axle is a Dana 44 and the front is a GM 10 Bolt. Both are plenty strong enough for 35" tires. I have also never read about or seen a failure of the stock front CV shafts.
Realistically 35" tires are about the largest you are going to be able to fit without going SAS anyways, so for all intentions the stock drivetrain is reliable if you intend to keep IFS.
As in what years are the best. I am partial the the older body style and interior. Steer clear of the GM Made 2.8 and 3.1L V6's, they are garbage and underpowered. Again, steer clear of the Automatics because they use the GM 4L30 transmissions, which are junk also. You want one with either the 3.2L SOHC or the 3.5L SOHC/DOHC engine, again these are made by Isuzu, they are powerful and reliable.
EDIT: This post may sound like I am bashing GM/Ford, but honestly I am not, I am simply stating things that make an Isuzu a bettery vehicle than the domestics which is what most people choose as wheelers. I am a Chevy person myself and infact I have put a SBC, TH400 and NP241 from a Chevy into my Rodeo.