Would the degree be valuable and in demand for the next few years (or decades)? Would the cost of the degree outweigh its benefits? Would I be wasting my time, or using it productively? And so on.
You can also work in the industry for a few years and then go for a masters. That has the advantage of showing you what you'd want to focus on and letting you take risks with your career prior to a masters (additional degree lets you reset a career). The disadvantage is that going back (especially if you are happy with your career) is hard. It's also easy to build up an industry arrogance (I found it in myself) and gloss over lectures when the way things are done in industry is different -- only to realize in a few years (when someone else builds a real-world system on an "academic" concept) that someone in academia had an answer to a problem you're only realized you're having now 30 years ago, but you dismissed as theoretical and impractical.
You can also do what I did: work full time and do your masters. Just be prepared to have a limited social life and less sleep for the next 3 years (you won't be able to take a full time academic load at all times while working). Not only does this mean you don't have to go into debt (and/or voluntary poverty) to get through graduate school, in many cases your employer can pay for part of this (although this limits you to companies big enough to provide tuition reimbursement).
1) Get a resume upgrade. I went from having a BA in Math from UC Santa Cruz to having an MS in Engineering (Industrial) from Berkeley. Personally, this kind of thing irritates the crap out of me, and I don't think I'd put much stock in it if I were hiring, but it can make a difference.
2) I got to investigate a new field. I suspect that if you already majored in CS, you probably don't really need an MS in CS. You'll learn something, sure, but I suspect you'd learn more just by going out there and learning. But if you majored in Math (like me), and discovered you really aren't good enough at math to make a career of it, you can obtain a more employable major through an MS. A lot of MS programs will allow you to apply a certain amount of upper division coursework toward an MS - so you've taken data structures, compilers, operating systems - almost like you've double majored, except now you get to say you have an MS (note: some of us are on to this trick - some grad programs are a little easier on their students and give them a lot more leeway to pursue a specialized interest - it's the undergrads who go through hell and get F's if they can't hack it, so I personally don't view an MS as necessarily "higher" than a BS in CS - it depends on what the undergrad studied and whether he/she filled these gaps in the MS program).
3) As I said above, you can pursue your own interest. MS programs tend to be much more flexible than BS programs. When I was in grad school, I pretty much learned to hack. My coursework wasn't all that interesting to me - lots of proofs about optimality in convex sets. I thought I was getting away from pure math, turned out I was right back in the land of proofs (that's Berkeley for you, other programs in IE are much more management-ish or engineering-ish). But I got really into writing apps, and that was my focus. Fortunately, the MS program is so flexible that you can get away with this - as an undergrad, I was far too busy with, well, proofs about convex sets (heh).
It's expensive, though. Fortunately, I got in before the price went way up - fees were only about $4k/year, and TA jobs kept the costs down. So I graduated with minimal debt and an OK experience. But I still gave up almost 2 years of earnings and had a small student debt at the end. If you can get this experience through working, eh, I'd probably just do that instead.
Also, you mentioned that some upper division undergrad CS courses can transfer to the Master's program for credit. Does this mean if I look into, say, Stanford's program and take as many of the courses as I can as an undergrad, then I'll be able to transfer those over (assuming I go to Stanford for graduate school)?
You probably can't apply a course that you took as an undergraduate toward an MS. Double counting like that isn't generally allowed. But hey, if you've already taken those courses, you can spend more time looking into something that interests you and specialize.