Its entirely up to opinion on that.
However, there are some common complaints. She has a reputation for having a voice so muffled its hard to hear and is a little difficult to tune for first timers. Her vocals vary too much per pronunciation over the notes and she is much more limited on her range then most Vocaloids. Her samples are also considered poor quality.
Saying that, she is a excellent partner to other female Vocaloids and can say more words then some of the other Vocaloids on say. If you've got the experience she is quite remarkable, however, you need to work with other Vocaloids first.
Part of the problem was that Sonika was intended to be a highly advance Vocaloid to begin with, with capabilities not seen in any other English Vocaloids. Some users find her too difficult to work with and are unable to achieve the results they want as effectively as they would with other Vocaloids with the Vocaloid2 software. Another contribution to her problem was that her voice was chosen to be soft and innocent, however this does not always work with her software language abilities.
Zero-G themselves have acknowledged the high level of fan complaints against Sonika, but do not feel she needs to be updated. Due to costs and promotion, it is also not cost effective to record the samples again. However they have since noted they will keep the complaints in mind for future Vocaloids.
For more details see Sonika's strength/weakness section.