I am in almost the exact same situation, except with 2 years of hindsight. I joined a startup straight out of college as employee #13 (at the time, it had about 8 employees in the office at any given time). It was profitable, I liked the people, and they were doing some pretty cool stuff.

Overall, I would do it over again but probably will not remain with them for much longer, if that makes any sense. Some observations:

The language choice will grate on you after a while. I dunno what the background of posters saying "Be professional. Suck it up." is, but I actually liked Java coding when I started, and have since learned to hate it. If you're going in with a strong aversion to Java & C++ already, it'll probably only get worse.

Flip side is that you do get better at these languages after you work with them a while, as long as you keep an open mind. There are tricks and tools in C++ to help you manage memory - you shouldn't be tracing pointers from new to delete. And Java's verbosity isn't as bad once you get used it and consider "for(int i = 0; i < array.length; ++i)...." to be a single map function that just has a really long name. ;-)

One question to ask that'll help you determine if the language will end up becoming a total drag: are they using it because the problem fits the language's strengths, or are they using it because they don't know any better and aren't comfortable with multi-language solutions? Every mainstream language is good for something. In my case, I was perfectly happy as long as I was writing IDE plugins and server software in Java, and then became very unhappy once I had to write a webapp. (IMNSHO, Java is irredeemably broken for webapps; the only sane way to do them is to strip out all the JavaEE bullshit and program on bare servlets, but then you get something that's nowhere near as nice as Python/PHP/Perl.)

There are plenty of things beyond programming that you learn from working in a successful (profitable) startup. OTOH, it's not rocket science. Basically, you can boil it all down to: 1.) Make something people want, 2.) Charge more for it than it costs you, and 3.) Fix problems now rather than later. Every failed startup I've been involved with has neglected at least one of these: every successful startup has done all three.

I dunno what you expect financially out of this, but be aware that founders get rich, late-stage employees get paid, and early-stage employees get screwed. At least in proportion to the amount of effort they put in. One startup I worked in went bust, taking my 0.1% of stock options with it. One that I interviewed at offered me about 0.01% of the company; I did the math, and calculated that at their expected $40M exit, I would get a whole $4000. I have no equity at my current employer. My cubemate was an early employee at CCBN, which was sold to Thomson for about $40M. He got $3K or so. I have another friend who was employee #35 at Stratus and later a VP; she was rich for a while when they ended up with a billion-$ market cap, but ended up poor again after the stock crashed. If you plan on being an early employee at a startup, you should do it for learning or for the experience rather than any perceived financial rewards.