monoCOG....
What can y'all tell me about them?
Great reputation. Very affordable. Best value in MTB in my opinion. 29er is the only format I can see myself buying.
Are they fairly lightweight?
No, it's a pig. Not a weight weenie bike by any stretch of the imagination.
How forgiving are they on trails?
It's all relative. It's a steel frame, so it will be more compliant than a full rigid aluminum frame. But, if you're used to riding a full suspension bike then it will probably beat you up a bit.
Recommended upgrades? Sizing? (I currently ride a Specialized Hardrock L (19") and it fits me perfectly)
Upgrades are a personal thing. The bike has house brand componentry. It's beefy stuff and has held up very well for me. But it's not light by any stretch of the imaginiation. At 250~, I'm pretty tough on bikes.
Sizing wise, 29ers feel like they fit a size larger than standard 26" bikes to me. My 21" frame Redlines feel like bigger bikes than the 21" Fisher 26" bike I used to ride despite the fact that, geometrically speaking, the fisher is the larger frame. Best to get out and ride one and see... kind of hard to size a bike over the interwebs with such limited info to work with.
MTBR has more info on the MC29er than you will ever need. Check it out.
I give it two thumbs up.