I' afraid to say all the brass. Otherwise it is quite useful, even nowadays, as long as you don't need staccatos or loud passages. I still use the ensemble strings to fatten my string sections. GPO has a nice sound that mixes well with other libs.
Truthfully, none of the sounds are that great except the percussive ones (harps, harpsichord, etc.) unless you have a lot of instruments playing at once.
But, I'd have to say the situation is similar with every library that I own. There are a few instruments that sound great and the rest are like mud. I have had to buy so many different libaries just to get enough to be useful.
Where Garritan differs from the others is that (a) you get a lot of sounds for your money, which increases the likelihood that there will be at least SOME instruments that you like and (b) its unique playing style, which for some people is a blessing and for others a major pain to work with. (Having to generate a Controller 1 value for every track just to get it to make a sound is a constant source of frustration and headaches for me.)
Given the price, and the fact that it is included with so many other applications, it couldn't hurt to have a copy. I find Jazz & Big Bands more useful than Personal Orchestra because of the variety of trumpets and trombones, with several different mutes for each.
Personally, I've found the trombones to be the weakest link in GPO. The other brass isn't outstanding, but it can be made to work without too much hassle. I've bought the WIVI trombones package and am considering the horns as well -- they supplement GPO very well and use the same playing style philosophy.
Speaking generally, if you use the right reverb and mastering, GPO still sounds great. I've played songs for people and had them guess that I was using some extremely expensive EWQL package -- nope! Just a $200 library. :-)