Spectrum line is your best bet, but Bachmann has also had the leading warranty in the industry for quite some time. The product is warranted for the life of the owner. If you have an engine manufactured, say, in 2003, and it goes belly up, just get an RA from the Company and ship it back to them. Chances are you'll get a brand new replacement....or a reasonable substitute if that model is no longer available.
If the item is older than one year (production date if you don't have a warranty card, or from the date of the warranty card), you now have to pay a modest repair fee. $15 for older HO/N, $25 for Spectrum and DCC on board/ready. $30 for O Scale, $50 for O scale Spectrum.
I have stuff from pretty much every "era" of bachmann production. The old Yellow box and White box was toy-quality junk. Spectrum was a step up at the time, but still not awesome.
These days, Spectrum is slightly better although most of the diesels are not up to today's standards. The Bachmann Spectrum electric locomotives are really nice, with the exception of the E60CP which is equally as nice but is not in the Spectrum line.
Bachmann's Silver Series is OK. The trucks are not interchangable with other brands due to the extra-large bolsters. The wheels are also not interchangable due to them being the shorter European (NEM) axle length. If you buy replacement wheels from Bachmann (the ones that come 12 to a blister pack) they will not fit because the replacement wheels are Intermountain clones.
The current Bachmann locomotives, which I call "Bachmann Blue Box" because they come in blue boxes, is OK. The detail is Athearn Blue Box quality but they have the Spectrum motors in them, are inexpensive, and run decently. Most of the blue box locomotives have DCC on-board. Bachmann also demoted their GE 44-tonner to "blue box" instead of Spectrum.