Got my son a set of these for Christmas and it's been a HUGE hit. I found a decent set with a few trains and tracks (Grand Central Station set) at Toys R Us for $50. Ever since, I've been going back and forth between Toys R Us and Target to find additional pieces to add onto the set.
Normally, my son, like most kids, gets tired of a toy after playing with it for a few days/weeks. With the train set, he has yet to tire of it.
PROS:
-Modular so you can start small and add additional pieces/trains.
-Tracks are easy to build so that even my 3 yr old can feel a sense of accomplishment in helping to build the train tracks.
-Remote control trains allow him to run several trains at once. My son is learning to manage the logistics of running several trains on the same track running back and forth when one catches up to the other or if they are coming to the same intersection at the same time.
-There are both remote control as well as "push" trains available which provides verstaility in type of play.
-Rugged. They seem to take a knocking and keeps on ticking.
CONS:
-The tracks require quite a bit of floor space. Unlike the Thomas the Train wooden sets, which he also has (but doesn't play with nearly as much), the GeoTraxx set will not fit on a Train table for any decent sized track. Expect to take up a whole room when set up.
-Battery usage. The remote control and the trains require AA and AAA batteries which can drain pretty quickly if play with often. Recommend rechargeables.
-Hard to find certain pieces/expensive for out of production pieces on ebay. Most major stores carry them (Target, TRU, Walmart) but once you start to build up the collection, you may inevitably start looking for pieces that are no longer in production. Finding these pieces can be difficult and expensive (ebay). I usually browse Craigslist to find someone selling a collection that their child has grown out of..but they sell quickly!
I've probably spent a few hundred on various pieces (trains, tracks, buildings, etc) but as much as he plays with them, I see it as money well spent. Much better than spending money on toys that he tires of quickly.
Recommended age is around 3 to 5 yrs but his 6 yr old cousins (1 male/1 female) both seem to love playing with them as much as my son.