I bought mine used.  I found what I still think was a pretty good deal.  It was a good deal because 1) the tractor was in great shape at a good price, and 2) the item was sold not because the previous owner didn't want it, but becuase after moving to a house with a small yard he no longer needed it.

GT 225: what does that mean?  GT means this is a garden tractor; it can do more than just cut grass.  It can also power a tiller, plow loose dirt (not just snow), or drag a disc harrow, cultivator, plow, etc without destroying the transmission.  Specifically it has the tuff-torq K71B transaxle, an upgrade from the LT series.  225: that doesn't really mean anything, other than this a 200 series tractor, similar in function to the old 200 series (212, 214 etc), and the smallest engine/deck combo of the GT line.  If you cut grass and anticipate doing a little landscaping or heavy yard work, this is what you want.  This is the tractor which isn't so big you look ridiculous mowing a half acre yard, but it's rugged enough to fill a lawn cart with 1200 lbs of firewood or topsoil and truck it around your property.

The GT225 came originally with a Kohler 15hp motor, which provides plenty of grunt to move this very heavy tractor around, but with hydraulic lifters, it taps a little when you first start it up after sitting for a few weeks.  No big deal. If you know anything about engines, you know even though it sounds a little ugly, no damage is being done, and it stops after a few minutes.  I used Mobil 1 synthetic and it seems to help eliminate it.  They fixed this problem by putting the 16hp Kohler in later models with solid lifters. 

The deck is the standard 42" convertible.  It's a good size for cutting a half acre.  Had I purchased this tractor new, I would have went with the next size up (the 48"), not because of the larger size, but because there are four anti-scalp wheels instead of three.  The 42" does not have a wheel behind the chute, so if you're cutting a delicate lawn like me (hybrid Bermuda at 1"), if you don't have the lift arms tuned to identically match the anti-scalp position (there are 4 positions for the wheels, unlike similar Cub Cadets which are not adjustable), you will get a slight scalp on the right side when you turn.  I'm overly picky.  My front lawn basically looks like a putting green.  If you're cutting fescue or bluegrass at 2" or more, you won't even notice.  The level of adjustability this machine offers isn't possible with the box-store models.

The deck is raised and lowered by a counter-sprung lift handle which is operated by your left hand.  it's fine if you drop the deck, mow your grass, and raise it when you're done.  If you're constantly raising it up and down to get over humps, you might want to look at a hydraulic lift (GX300 series).  Unlike the old 200 series, there is no implement lift in the back, so you can't get a good sleeve hitch.  I have seen a couple aftermarket models which incorporate their own lift handle, but now you have lift handles all over the place--a little cumbersome.

The GT series has the big wheels.  That's a good feature.  If you have a slightly bumpy yard or need traction on hills, you need the big wheels. 

The GT series has a much more rugged frame than the no-longer-produced G series (which are the classic flimsy tractor with a big engine, marketed by John Deere at home improvement stores to cater to people who want cheap lawn mowers).  It's fully welded and has a bracket on the rear for suitcase weights.

This tractor has an electric PTO, which means you just pull a switch on the dash to engage the deck.  If you pull up on the switch before backing up, you can override the Reverse Implement Off (RIO) feature, and mow in reverse.  Otherwise it will shut off when you back up.  I've thought about disabling it, which would be very easy, but with a lot of little kids in the neighborhood, you never know who will be behind you when you mow backwards, so it's a good feature to slow me up a bit and keep me looking around. 

The high-back seat is very comfortable, has its own elastomeric suspension (adjustable for weight) and moves fore and aft like a car seat.  There is also a cupholder.  But if you have thoughts of drinking a cold beer while you mow your lawn, forget it.  The vibrations make your drink go flat in a matter of seconds.  You'll have to stick to sweet tea (or whiskey).

On the right side is the cruise control.  It works great.  But strangely enough it doesn't disengage when you step on the brake. 

My tractor came with the 7 bushel bagger (non-powerflow).  I was originally completely unhappy with it, but after changing the blades, I've had no problems; no clogs and nothing left behind.  Apparently the old blades were not the high-lift blades.  Either that or they just lose their ability to move air when the bent-up tabs on the back get worn from a season of mowing my sandy yard.  

I also got a 10 cu ft cart with the purchase.  it's okay.  I had a Brinly 12' cart which I sold with my Cub Cadet.  That one was built a little more rugged, even though it sells for less (but it wasn't green like the new tractor, so I got rid of it, sadly).  I don't think it had greasable axles like the JD cart, however.

The most important feature of a garden tractor is the strength of its transmission.  Most JD garden tractors (and many Craftsman, Husqvarna and others) have transaxles manufactured by Tuff-torq.  You can find specific information on each model on their own website.  The engine size is the least important factor.  This tractor has a 15 hp engine and it mows significantly faster and better than my previous Cub Cadet 1000 series which had an 18hp motor.  Most of the tractors you see have engines which are far more powerful than the transmission can handle.  I've hauled my 10 cu ft trailer heaped over with sand up hills and never saw a need for more power.  The transmission, the tuff torq K71, is basically the same K71 used in the GX series.  The only difference is the K71 in the GX has a differential lock--definitely a nice-to-have, but expensive.

On the topic of horsepower... after a recent ribbing by my neighbor, the proud new owner of an 17hp John Deere X300, about how his tractor is slightly more powerful and therefore better (aside from the fact that the wimpy K46 transaxle can't pull anything), all debates ended after a 200' drag race in my back yard in which my single-cylinder Kohler beat his V-twin Kawasaki by at least six tractor lengths.  But that X300 does purr like a kitten... I'll give him that much.

These machines are pricey (you'll pay more for a used GT225 with a 42" deck than a brand new Troy-bilt with a 42"), but if you take your time and keep your eye out, you'll find a good deal.  I bought this tractor with 100 hours on it, in pristine condition, with the bagger and cart, for $2,000.  New, this combination sold for $5,000 (comparable X500 starts at $5,600).  If you don't know what to look for and don't have the means to transport one yourself, go to the dealership and buy a new one.  But if you're willing to take a little time educating youself and searching eBay for a good deal, you will be rewarded.  You'll end up with a much more capable tractor for your money than if you'd bought a new one at the sticker price.

Good luck!