The Muzzy pipe is a popular mod for the EX250. There are a few things to consider before you put one on, though. There is more to putting a Muzzy on your bike than pulling it out of the box and bolting it in place. Here are a couple things to consider:
The melting problem
Muzzys have very tight clearances on the EX250. If you don't do something about this before you ride it, your lower fairing will melt. Some people suggest putting a couple extra washers on it; this will bring the clearance out a little. You can also use two 1 inch plastic spacers on either side of the fairing at the bolt point. You will have to get a longer bolt, though. A 6x30mm should fit. This will hold the fairing out more.
The best thing to do is trim the fairing as shown. The Muzzy pipes are a very tight fit and will press against the lower. Making a heat shield won't help much. A Dremel tool will take care of the problem in a jiffy.
One method is to let the exhaust melt the fairing back a bit to mark it, then come back later and trim it away with a dremel with a cutting wheel. You can't see the mod to the fairing without laying on the ground and looking.
Another thing you can do is wrap that small section of pipe with exhaust tape from any auto parts store. It will prevent conduction of the heat to the plastic. DEI header heat wrap can be found at hot rod shops. You may want to do some research first, as this stuff has been known to make pipes corrode quicker.
Hints, tips, & opinions
Do I have to remove the centerstand to install a Muzzy exhaust?
Understandably, most people want to keep their centerstand when they put on a Muzzy system. Centerstands are very useful.
There are a few problems with this, however. First, the spring that holds the stand in place when retracted rubs the Muzzy's larger head pipe. It just touches the pipe when the stand is down. When it's up all is fine, so this shouldn't be a problem for most people.
Second, there is no "stop" for the stand anymore when retracted (since the left canister is gone); there is nothing to keep the spring from pulling the centerstand up into the swing arm and drivechain. You could fabricate a bracket to hold it in place. One example looks like this:
Lastly, the Muzzy is designed to increase performance a bit, sound good, and look good. One way it looks good is by really cleaning up the back of the bike; with the stock exhaust cans gone the swingarm is fully visible. If you leave the centerstand on, it will be quite an eyesore. Totally opinion, of course.
If you decide it's more trouble than it's worth to keep it, you can buy a nice swingarm stand for about $70-100, which will let you do most of the maintenance the centerstand allows. The only time you'll probably still use the centerstand is when you're doing suspension work; the stand can be loosely attached temporarily with two 17mm bolts, and put back in the parts bin when the work is done.
How much difference does the Muzzy exhaust system make?
The Muzzy is loud, but it has a deep tone; I don't think it is obnoxious. A good helmet keeps most of the noise out. Oil filter and drain plug access is fine. The stainless steel head pipe looks great and fits perfectly. Less weight + more power = a faster, better-handling bike.
Muzzy is a great pipe - but it's going to cost you more cash than the performance increase is worth.
Not only do you have to shell out $400 + for the pipe, you need to rejet ($), get rid of your center stand (buy a work stand for doing maintenance)($), cut your lower cowl (so it doesn't melt – it's not a perfect fit), and repack the core occasionally (time)($).