You sure can make a career out of 135 freight. I know of many people that have or will be career 135. Generally speaking, the people that seem to be the happiest with that decision are the FedEx feeder guys... the people working for corporate air, mountain air cargo, empire, etc. I don't really understand how much competition there is between those companies, but it seems minimal. The pay raises are steady generally, FedEx owns all the planes so they have a vested interest in you not being an idiot (you're still freight so you'll see your share of weather- but from my experience, them canceling a flight due to weather is a lot more 'acceptable' than by many other operators), and overall those pilots seem to be happy.
Personally, I fly UPS 'feeder' freight- scheduled routes. It's all contract work. It's a great job, I love it, and I'll probably continue doing it for a few more years. But, because it is all contract work any company can lose their contract at any given time (usually requires a 30 day notice- but that ain't much). Many companies have lost many of their runs in the past... Suburban air freight lost many of their runs to Ameriflight last summer. So, it happens. That's one of the drawbacks of working at some of the smaller (and even bigger companies-- Ameriflight lost their Billings routes to Alpine not that long ago) scheduled 135 operators. It is nice though... you know when you'll be at home 95% of the time (there is always the chance of a charter or you getting diverted due to weather... so there never is a 100% guarantee of being at home). But, overall it's a lot of fun. I know a few people that will make this a career, but it is less stable than the FedEx feeders.
Unscheduled freight is a crapshoot no matter what. It goes with how well the economy is doing. Lately, it has been pretty rough for the unscheduled companies. Some are doing well... many are not. The economy will pick up, but until then it's going to be a little rough for many of those operators. Also, with the unscheduled freight, there are no guarantees you'll be home much.
Good luck figuring it out. It's a fun road... but it does get boring flying the same route every day.