Here's how I decide if a username is good or not:
1. Username is gorman
2. Username is gorman but the o or a are swapped to another vowel or the numbers 0,1,3 or 4 (since they look like vowels).
3. Failing that, at least let it be an immortal animal.
4. Or at least several animals...
Examples of good usernames:
gorman, girman, g0rmun, gormon, garm4n
Examples of bad usernames:
siku, ryan, tiner