I've heard both wonderful and not so great things about MTH trains.
I personally don't consider them to be worth their price...the added features don't come close to making up for the added cost beyond Spectrum or Broadway Limited.
I don't care for Athearn's new stuff as it is too specific to prototypes which have been widely produced and are of no interest to me.
MTH has a bit of a bad name in HO because of their lack of desire to place nice. They have a proprietary DCC system which really ticks people off. In HO, all other DCC components are compatible with each other and this has caused it to really grow (the standards were based on Lenz's system in the 1990s). MTH locomotives have diminished capabilities on normal DCC systems...the most notable was that their K-4 could only achieve a max speed of around 50mph on a normal transformer due to its higher than normal motor voltage.
What type of locomotives would you like have purchase? Do you plan to model a specific prototype, a free-lance road, or just have a random collection of locomotives?
If you are planning to jump into HO, I'd recommend starting out with Spectrum steam locomotives and either MRC or NCE DCC. I don't think anything really compares to them on the value basis. Spectrum steam is typically on par with the stuff that cost 2x what it sells for, and much of it is sufficiently generic to be useful on both free-lance and road specific layouts.
Generally, ignore the plain B-man stuff...the Spectrum is much better...
http://www.bachmann.co.uk/price1.php?prod_selected=americanho
Their 2-8-0 is what launched the modern era of HO steam locomotives.
Athearn is classic for diesels...their Genesis line is competitive with Kato and Atlas...their standard line has long been the best value in diesels.
http://www.athearn.com/
Athearn also offers the old MDC Roundhouse line...but they are typically not up to modern detail standards.
Broadway Limited is the HO equivalent to MTH (although a recent court case has resulted in some of BLI's stuff going to MTH due to an issue with BLI's Korean producer).
http://www.broadway-limited.com/index.php?p=catalog&parent=1730&pg=1
Walthers now operates the Life Like Proto 2000 line...which has a nice slate of quality offerings (plain Life Like is toy stuff).
There are also kits...Bowser is the source for Pennsy equipment...as well as a few others...but they can be detailed to look like brass importants and pull like nothing else, but they are not modern in technology.
http://www.bowser-trains.com/Main HO.htm