I collect rucksacks and I feel that the Alice pack is one of the best external frame packs out there. In the winter, I would opt for an internal frame pack which saves a bit of weight and bulkiness, but the other three seasons will find me with an ex frame. As for frame packs, most frames are too long to be comfortable and balance the load about where your neck is. You can't look up, and when you lean over, you fall over.
The alice frame is stout and holds your load lower and further out. This puts the weight on your shoulders and kidney pads. This also puts space between your back and the pack for better ventilation.
The only things I change on my alice packs are fastex buckles on the vertical straps and some webbing to make a handle at the top of the frame. Aftermarket shoulder straps will improve comfort especially with a sternum strap, but I have yet to actually NEED one.
On a side note, a lot of guys in green replace their frame with a plastic one.