From a technical standpoint, the G11 will be one of those "could have been" designs. It was the pretty groundbreaking in many areas and it must have been absolutely heartbreaking for the engineers when it was shelved after ~25 years of R&D, on the eve of entering mass production no less. However there were some issues that I think would have come up in real world use that would have needed to be addressed. The 4.7mm ammunition would have likely had worse wounding characteristics than the current 5.56 fired from a shorty M4. The three 45 round mags in the gun would have come in handy in a quick firefight, but in sustained fire it would have been a real PITA to reload those things from the 15 round "cartridges" that soldiers were to be issued. Finally I've read conflicting reports as to how well the G11 actually stood up to use. Some say that the caseless ammo burned extremely cleanly and caused little fouling, others claim that it gummed up the gun worse than any M16 and suffered from significant throat erosion. Dynamit Nobel claimed that the caseless ammo was waterproof while others claim it was very sensitive.
Interesting fact: Apparently when the G11 ws finally dropped, H&K Germany had to destroy most if not all of its preproduction samples. Some of the few remaining G11s are supposedly in Canada, after their use in NATO trials 10 years ago.