Most any device that uses 256 bit or greater AES encryption is considered “safe” because the bit length is long enough to keep modern computer’s busy for years running brute force decryption. The problem is the misconception that once a HD or SSD is encrypted; the only way to acquire access to the data is through brute force attacks. The responses thus far to this question have been exclusively taking into account the software/firmware vulnerability side of the encryption.
It indeed is important to understand the correlation between FDE, the BIOS, and ATA passwords, but it’s more important to understand that locks only keep honest people out. A crook or law enforcement who wants access to your data will not spend time trying to guess or brute force your password. The fact is, a simple off the shelf hardware key logger (meaning on that installs between the keyboard and computer) will instantly begin logging keys the second power is applied, irrelevant of the execution of the bios. A simple test setup consisting of a key logger, a keyboard and an external power supply will demonstrate the logger capturing key strokes even without being connected to a computer.
There are dozens of techniques used to steal your password. Key loggers, hidden cameras focused on your keyboard, phishing, eavesdropping, breaking your arm, social engineering, rummaging through trash. Don’t forget about a subpoena to your ISP, email provider, bank, eBay, Paypal, credit card company, etc. requiring your password on file. Also be aware that to date, all encryption can be detected on all HDs and SSD even Truecrypt. This means that once it’s determined that your drive is encrypted; you can be compelled in court or through discovery to provide your password. Of course you always have the option of exercising your Fifth Amendment right which may or may not protect you from self-incrimination.
The question “How secure is Intel SSD Encryption” can’t be answered by only considering its hardware AES encryption. Data security is much more than encryption and you should never store anything on a drive that could incriminate you, even temporarily because tools exist that can can recover things you delete. Keeping your hard encrypted, keeping it in a secure location, having a unique password that you don’t use anywhere else, not sharing your password with anyone, awareness of social engineering, visual review of your working environment, a watchful eye on your surrounding for a crook, thug, or investigator, and routine inspection of your hardware for loggers and such will greatly increase the security of your data.