Is 'Some of y'all' grammatically correct?

Basically "y'all" is just a casual, familiar term for "you", so "some of y'all" is essentially the same as "some of you".

(And, per livresque's comment, it should be noted that "y'all" is normally considered plural. For this reason it's somewhat useful in conversation, as it makes it clearer that the associated context is being addressed to a group, and not just to an individual. This no doubt is a big reason for the term's persistence.)

"Y'all" is normally, in the US, considered a "rural" usage, and it's used in country music (such as Y'all Come and movies/shows (such as the Beverly Hillbillies) that portray rural characters. However, since it's considered "illiterate" it's passing out of fashion.

In Britain we never use y'all (unless we are imitating a Southern American accent).

However, in Britain, it is perfectly idiomatic to say, for instance:

Are you all going to the party? (this makes it clear that we are speaking to everyone present and not to just one person)

"Y'all" is not a vocabulary item in in standard Modern English. But it is a term in the 'southern' US states and AAE (African-American English).

'Grammatical' can mean two things: 'Does it fit a regular pattern in a given context?' and 'Is it socially acceptable?'.

Ending sentences with a preposition is considered socially unacceptable in formal writing, but when we speak we do it all the time.

Using "y'all" as a second person plural is not used in standard English, but is perfectly grammatical (rule-based) as used in varieties of English spoken towards the southern states of the US.

So your fragment

Some of y'all

is perfectly fine in Southern American English. It is not part of standard US English but is perfectly well understood by speakers out side of the south.

Of course there is no essentialism and all concepts are fuzzy near the edges.

Y'all is a dialectal form of a second-person plural pronoun. Therefore,

Some of y'all have too much free time

is as grammatical as

Some of us have too much free time

or

Some of them have too much free time