How secure are current deterministic wallets?
It is secure.

128-bit seed is at this time very secure. It is lower than a regular bitcoin address which is actually RIPMED-160(SHA-256), giving it 160 bits of entropy.

Brute forcing a 128 bits of entropy at this time would take around 1,440,000,000 years if every computer ever made was working on this single problem.

Like most of these algorithms brute forcing is not the issue. Side channel attacks are usually the danger or if an error is found in the implementation, which none have been yet.