The biggest problem is that we don't understand how the brain works well enough. It's quite complex and pulling out useful information is very challenging. Machine learning algorithms can be ran on neural data (fMRI/EEG/MEG) to identify some simple words or detect specific intentions/reactions to stimuli, but we definitely aren't getting this information at a rate which is anywhere near faster than current human interaction methods and with nowhere near the accuracy (in real world settings) we would need to be practically useful.
My concern is that we haven't yet gotten smartphones right. Most of us love and hate them. I'd love to have something like parental controls on my phone which I could use to limit what I can do with it: set limits on my usage, control which apps I can use, and limit what I can do with them. There's a variety of enforcement mechanisms ranging from putting in a longer random password and not writing it down, have my spouse set it up, have it setup initially and then come up for review every three months...
But imagine if we all had something similar, inescapable, engaging, entertaining, and utterly addicting in our heads. Yikes. Imagine if you've got notification overload...In your head. Yikes.
No thanks. I'm still trying to get a handle on my damn smartphone addiction.
Also, I feel like I think way faster than I can type. I can visualize the piece of code in my head before I even type it out. Imagine no more spelling mistakes :)
Brain surgery already seems very scary, as you have to stay awake during it. Then when they come out with new hardware generations, go under the knife again? Brain surgeries also are expensive. But I could see how something like this would make me 100x more productive than I am now. So if I created something really cool using my new special abilities it would probably pay for itself.
So I see a lot of upsides and downsides. I don't know if this would be something I'd want to be an early adaptor of, though. Imagine if the company supporting it went out of business, now you have this unmaintained piece of hardware in your head. If something like this was perfected, I feel like this would be an asset to have just for the productivity advantages.
Requisite plug for Echopraxia by Peter Watts :)