My Craziest Desire

The strangest thing I've ever wanted for myself is a head computer. A cyberbrain, if you will. It's technically possible, I believe Dr. Kevin Warwick will have one for sale within the next ten years. It's too early to be sure of the details yet, but let me talk about what I assume the advantages and disadvantages would be.
On the plus side, I should be at least able to offload all mathematical operations onto it. Let's say I'm shopping at the supermarket, and I want to know which can of corn is a better deal. One can is $0.90 for 8 oz, one is $2.50 for 30 oz. When this has actually happened, I am lost in thoughts for minutes as I attempt to make the necessary divisions. But using a computer (and my fingers), I can have an answer in 30 seconds, using a calculator program, "bc":

~ $ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
scale=2
.9 / 8
.11
2.5/30
.08

The second option is the better deal, at 8 cents per ounce. This would be even faster, since I wouldn't have to bother typing it, I would just think it and it would happen. I would also be able to write my blog from anywhere, and interface computers with no human interface devices. Don't need keyboards, mice, or monitors anymore.
In the event that brain-calculations and computer-calculations are compatible (which is dubious), I would gain a thousandfold increase in mental capability, sharpening my senses by significant amounts, and generally being a superhuman genius.
On the downside, this would involve the most invasive surgery possible. The device would need to be wired to my nerves, possibly even implanted in my brain through a large sawed hole in my skull. (Which I need like, well, to pardon the pun, a massive hole in my head.) Also, massive software security would be needed lest it be useful as a torture mechanism. It might be possible to control me through the device, or if not, drive me insane through incessant voices in my head. It may, depending on the construction, be possible to use the device to read my very thoughts. I don't give myself the pretension of thinking that anyone would give a crap about my personal thoughts, but if this becomes common, I can definitely imagine entities that salivate at the thought of being able to instantly read the mind of anyone they deem to be their nemesis.
Lastly, I don't know enough about medicine to know about how the immune system deals with foreign bodies. Would it grow scar tissue around the computer? (I don't want scar tissue in my brain!) Would the immune system attack the computer, ignore the computer, or attempt to cordon it off? Would an electrical malfunction in the computer result in me suffering an epileptic attack? (I don't have epilepsy now, but computers involve stronger currents than the brain usually has, and should they "leak...") If the computer's code should malfunction, could I be stuck with an annoying input that never goes away, or would I have some way to reset or turn off the computer?
Maybe this isn't worth it.