I've been down the road of the wallyworld bike. I've ridden 10k dollar mountain bikes, and a lot in between.
I am able to discern the difference in how an entry level, bike shop quality full suspension bike rides, Vs. the very same manufacturers higher end offerings. I can feel the difference in a couple degrees of head tube angle, or a shorter or longer chainstay.... I understand bicycle geometry. I also have about an 8 year background in welding and fabrication and will soon be building a frame jig to begin building my own bike frames, as such I've been researching a lot, and talking to a former professional frame builder to answer my fabrication and frame jig questions. I understand the science behind bicycles.
Regarding the blackcomb, the ditch, the and the similar bikes from Schwinn... they are basically the same thing with slightly different components, and slightly different geometries, all of them copy old suspension designs. Not a single one uses a pivot bearing anywhere on the frame, its all bushing style connections full of stiction and/or play. They have a severe amount of lateral flex, what this means is, anytime you stand to climb or spring you are flexing the frame laterally, not to mention the suspension bob due to the rear shock having NO dampening whatsoever. The dept. store schwinn I bought to start riding again after a hiatus of several years would experience tire rub if I stood up to sprint or climb. The frames aren't dangerous, or more prone to breaking, mainly because they are severely overbuilt, heavy gauge tubing is used on all of them. The hazard for "off road riding" is mainly the fork, and wheels, as they aren't designed for mountain bikes, but just as general use products for bikes that might see some gravel roads, but mainly will roll down paved bike paths. For most people who buy these bikes, this is fine... They get out of them what they paid for them, and typically they last a few years and then get thrown out. Summer time at the bike shop is a parade of people pulling dept. store bikes out of storage and wanting to get them tuned up so they can ride... then they hear 80 bucks for a complete tune and junk the bike, starting all over again, "I only paid 150 for it!"
The general attitude here is pretty much the best one to have:
If you already have one, great, ride it, fix small things if they break, but don't invest much in this bike, and when it craps out, if you are really wanting to get into this sport, buy a bike shop quality bike, and keep at it.
The attitude on used expressed by Irishmongooserider: just like you don't buy a used car from a private seller without having a mechanic check it over, the same should go for high end mountain bikes. the seller should agree to having a bike shop check it over, maybe meet you there with the bike. most shops offer free estimates, so have them check it out. it lets you know what you're getting into, as well as gives you bargaining room on the purchase, maybe the seller would knock off the cost that the shop gives you on the repairs. Don't forget though, if you are going to use a shop's services like this, you are sort of implying you plan to spend money with them, at the very least, buying whatever consumables you need from them, even if you can get them cheaper online and do it yourself.. maybe spend the extra 5 bucks on the new chain at your LBS instead of online, and buy your lubes and such there as well... MINIMUM, if you want that shop there to help you when you're stuck, you need to show them loyalty or they might not be around next time you need their mechanic.(not directed at you irishmongoose, just a "royal you").
Dept. store bikes can be upgraded to a safe level for basic trail riding/XC. They will not be as pleasant to ride, will not handle as well, will not climb as well, will not descend as well, and will not last as long. but doing this will cost you as much as a good used bike which will last you longer, and perform better, making your experience more enjoyable. Additionally, there are factors in these low quality bikes that start to become safety issues all their own, excluding complete failure of components, there are aspects that a better quality bike wouldn't suffer. For example, the pogostick rebound of the rear shock on these bikes can lead to OTB crashes because there is no way to slow it down. The flimsy fork and wheels can cause difficulty navigating technical rock gardens, or rooty sections of trail, causing deflection of the front wheel, again, potentially causing OTB crash, or simple front tire washout, point is, you're more likely to crash on inferior equipment then you are with better parts. The list goes on, but unless you are a VERY casual rider, who ventures into the woods only a few times a year, and mainly rides paved paths and some gravel/dirt roads, riding less than once a week, You should not look to make a long term ride out of a wal-mart bike.
this advice is an echo of posts I've made before, and the product of having been through upgrading a wal-mart bike, owning a "pro line" mongoose, working in a bike shop, and having metallurgical and metal fabrication knowledge. It can get you into riding, and that is awesome. And certain parts CAN translate over to a new bike. But the value in a bike shop quality bike is something that cannot be matched by dept. store mountain bike style bikes... and we didn't even get into frame sizing issues and proper fit, and how that relates to safety, comfort, pain while riding, etc.
This post is already a novel, but I hope it reaches people it can help to make a better decision regarding their new bike purchase, or what to do with the mongoose blackcomb they just got at a yard sale for 50 bucks to get into mountain biking with.
TLDR:
dept store bikes are not worth lots of upgrades, the difference in the frames is hard to understand without a fair bit of knowledge of bicycle frame design, and material specifications, but just understand they are on average, at least 150% as heavy as a comparable frame on a bike shop brand bike, are more flexy, come only in one size, and are not as strong, though not unsafe. the weak parts are fork and wheels, and are dangerous if they fail. Buying used is fine, but get it checked by a bike mechanic before buying. Dept. stores sell toy bikes, understand that you are getting a toy bike meant for kids to ride around the neighborhood, and plan your riding, and expendatures accordingly.