Which word you emphasize depends on what you are trying to say.
"So what can you do?" puts the emphasis on the other person's ability or lack of ability. Like if you got a new assistant, and you assigned him some task and he did it very badly, and so you assigned him another task and he did that very badly also, you might plaintively ask, "So what can you do?"
"So what can you do?" puts the emphasis on the other person as an individual. If several other people have just volunteered to perform whatever tasks and one person is sitting there doing nothing, you might ask him, "So what can you do?"
Update After Reading JR
Good point, let's consider all possible emphases. In context each might have other meanings, but obvious interpretations of emphasizing each of the remaining three words are:
"So what can you do?" Given what has gone before, what can you do now.
"So what can you do?" Of the possible tasks that someone might be capable of, which are you able to do? (This might mean of all possibl tasks in the world, or of all tasks that need to be done right now.) Pretty similar to emphasizing "can", I think.
"So what can you do?" What are you capable of actually performing, as opposed to just talking about it.
Grammatical Correctness
I believe both are grammatically correct. If you take out the apostrophes you can write:
It has been a while since I have sat at a dinner table together with a family.
and
It has been a while since I sat at a dinner table together with a family.
Both seem fine to me.
Which is More Commonly Used
The best I could come up with for which is more common turned up the following (from Google books searches):
I've * Variants
"been a while since I've" OR "been a while since I have"
About 158,000 results
search results
I'd * Variants
"been a while since I'd" OR "been a while since I had"
About 50,800 results
search results
I * Variants
"been awhile since I" -"I've" -"I had" -"I have" -"I'd"
About 11,600 results
search results
You can also see a similar story unfolding by looking at google Ngram viewer using "been a while since I *".
Subtle Meaning Differences
As for the subtle differences, I think it boils down to the tense of the verb to sit
.
I sat
is the simple past and is used to
express the idea that an action started and finished at a specific time in the past. Sometimes, the speaker may not actually mention the specific time, but they do have one specific time in mind.
http://www.englishpage.com/verbpage/simplepast.html
I have sat
is the present perfect and is used to
say that an action happened at an unspecified time before now. The exact time is not important.
http://www.englishpage.com/verbpage/presentperfect.html
Best Answer
They technically mean the same thing (and both are grammatically correct).
There is a difference, but it is subtle. It's more clear in spoken English, or at least easier if you add an emphasis where necessary (which I have done below).
Plain and simple, the thing works; no implications here.
Think of it more as "You would think it wouldn't work, but it works." Perhaps other products in the same category don't work, and this dispels any disbelief in the product's ability to work.
Even more emphasis on the fact that you think it wouldn't work, but it does.
And if you want to add even more emphasis (but is on the brink of grammatically incorrect, and overuse of this might lead people to stop taking you seriously):