If you need themes, its works well enough
Theres nothing especially wrong with Alarm Clock HD. Just remember that iOS ships with its own "Clock" app which, contrary to what you might expect based on the name, includes a perfectly functional alarm clock. As such, unless you really desire the additional control over the theme, Id recommend sticking with that. If you do, however, be aware that even with the brightness turned down all the way, its surprising how much light the standard themes put out; for me, I had trouble sleeping with it on, unless I turned it away (which, in many ways, defeats the purpose).
JeremyCaney about
Alarm Clock HD, v3.0.4