Originally named 'The professors cube', the 5x5x5 is designed to be a challenge for cubers who have mastered the 3x3x3 and 4x4x4, it is not much harder than a 4x4x4 and does not have PLL parity, it is generally solved the same way as a 4x4x4. Modern 5x5x5s are almost all brilliantly designed and definitely worth a try. The best speedcubers can solve the 5x5x5 in around 40-45 seconds on average.