The 426 is a great bike! i own a 02 Yz426. I rebuilt it from literally boxes and did a fully built the motor. ive owned it since i was 16 and am now 19. ive had no major issues or complants since i finished the build, just tons of fun!
the only things that these bikes have a bad reputation for are overheating, hard to start, and their abit heavy cause of the steel frame and being a big bore thumper.
I have never had an issue with overheating, just dont idle the bike like you would a two stroke, since I'm in SO Cal and this summer was pretty hot, just to be on the safe side I switched to engine ice coolant and picked up a boyesen supercooler water pump kit, the bike runs very cool and still has never been close to overheating out in the mojave.
as for being hard to start, as long as you know "the starting procedure", keep your valves in spec (like any 4 stroke) and your carb is clean/jetted properly (im just useing stock jetting) mines fires first kick.
also to help ease starting look into the 03 yz450 exhaust cam mod, or pick up a "hot cams" exhaust cam. they have auto decompression and allow u to stomp on the bike like a two stroke to fire it up makes it much easier. (next mod on my list) as well as a great power increase if you get the intake and exhaust cams from them.
as for being heavy... you get used to it, also im 6" 1' and weigh around 200lbs so man handling this bike around is no problem for me. It's not super heavy, but when you compare a two stroke and this thing its totally different.
For being one of the earlier generation 4 stroke racing machines this thing is an excellant bike and I stand behind mine all the way, the powerband is unreal and rides more like a 2 stroke then a 4, pulling much harder in the higher RPMS thanks to the titanium valves.
(2002 modle only, other years had steel though im sure they still pull hard in the higher revs as well.)
hope this helps, if you have any questions lemme know