It depends what you're expecting. There will be outages from time to time; common problems are things like other users DoSing themselves (and, because you're on the same server, you too), other (poorly-coded) sites tying up the MySQL server for long enough for your script to time out, rogue logging scripts filling up the shared storage space -- that can happen in the relative blink of an eye -- that sort of thing. Oh, and there's always the off chance that somebody might get slashdotted/reddited/HNed, which will pretty much overload the server for everybody.
In a lot of ways it's like any shared hosting, but because you have fewer resources available and more accounts on each server, there's a little less room for (recoverable) error. If you're running a fun/hobby site, it will likely be good enough. If the server's busy, a refresh will usually clear the problem (and users are more likely to blame their own ISP than the server they're trying to go to). But if you're trying to run a business and can't take a chance that a customer will use the back button and click on the next search result, then you'd probably want something a little more consistently responsive and with staff monitoring full-time. Illuminated would be better than regular or Prime Free Hosting, and Premium would be better than Illuminated. Again, if it's a business thing, going to Premium is really a no-brainer -- the most you can lose is $142 if it doesn't pan out, and that's a really low business investment as these things go (even taking into account economic realities outside of North America and Western Europe). And if you're running a hobby-scale forum, then the only people who you're going to lose to occasional short outages are the insufferable trolls anyway.