Pros:
This course has a couple of the best holes in the state. As have been mentioned before, holes 2 and 3 -- especially from the longs -- are awesome, memorable, challenging beautiful holes.
The teesigns are great, two concrete teepads on most holes is a plus. The longs play harder than probably any other course in the DFW area, which is good, because DFW needed some courses that are closer to blue/gold level to challenge players to expand their game.
The mix of wooded and open holes is good. It has above average elevation changes for a Texas course. Mix of short and long holes, with everything from deuce-or-die shots to a true par 5 (hole 3 long) that requires accuracy, distance and placement.
Cons:
Other people have mentioned the drainage problems, so I won't go into those -- they are a problem, but the four times I've been to the course (in winter, fall and early-spring) drainage hasn't been a big problem -- certainly no worse than other courses in the area (better in many cases). So that's partially a tolerance thing. Do I wish the course was dry and friendly -- yes -- but can I live with some swampy areas on the course -- yes. Just don't bring your best shoes and keep an extra towel on you and you'll be fine. However, the basket for hole 1 is in desperate need of moving a little away from the water because the slope is so unforgiving that it eats a lot of discs on what were great throws. Bad luck should not consistently cost players a disc -- even when they throw safe shots -- yet this hole does.
The bigger problems I have with the course -- that keep it from getting a higher rating -- are that a lot of the back of the course just gets a little laborious and doesn't force enough different throws. The back nine is more of a traditional "Texas"-stereotype course -- big hyzer backhands will get you everywhere it doesn't force players to really improve their games or their shot selection.
My pet peeve on the course is the horribly-marked OB on hole 17. The signage shows a row of planted trees that are the OB-line. Yet the hole has 20-30 trees in several lines (you can connect the trees in at least a dozen different lines), and it is impossible to tell from the signage what was intended by the course designer. This makes a huge difference in strategy for playing the hole. I think the artificially marked OB is a great idea in making the hole more of a risk/reward shot, but if you are going to do that, you MUST mark the OB trees clearly or you just cause innumerable headaches for every group who tries to play the hole.
Other Thoughts:
I feel like I'm echoing a lot of existing sentiments, but I figure the repetition is worth hearing for people interested in playing the course -- and hopefully for the parks people and folks who maintain the course as well.
For people interested in playing the course, it is worth the drive out to Rockwall, and the course does have some very memorable shots. I am usually a fan of playing the longest, hardest layouts possible on a course, but in the case of Harry Myers, I would change my recommendation: play the shorts and you'll be much happier (except maybe holes 1,2,3 -- play those long). The shorts flow better and are less mentally taxing than the longs and really make for a much more fun day of golf.
For most players, the longs will just be frustrating because with the exception of a couple of holes, the longs are just far enough and open enough that shot selection becomes nonexistant: you just take your biggest driver and bomb it RHBH and then go for the lay-up par-3. That's just a silly way to play golf and lessens the joy and beauty of the game.
Play the shorts where your shot selection still has to be careful, but where control and placement off the teebox at least comes a little more into play. The longs have some great potential to spread out the pro scores at tourneys, but for most players, they won't be worth the frustration on a day-to-day basis.
So in short: definitely worth playing if you live in DFW, just to work on different elements of your game. If you're visiting the area, its worth playing just to play holes 2 and 3 (play them a couple times each and enjoy), but don't expect to be "wowed" by the rest of the course.