Straight Talk is actually a rally good provider, because you can choose, if you want to use Sprint, Verizon, T-Mobile or AT&T network with them. (This depends on your phone, of course.) If you choose the T-Mobile or AT&T network, it even roams to the respective other one, if your original network is not available.You can switch between the networks at any time, which is nice. (For this, you may have purchase a SIM kit, though.)
Generally, it makes most sense to bring your own phone. This way, you can be sure to be on the network you want. It's important to make sure that your phone supports all the bands of the network(s) to enjoy maximum coverage.
If you want to buy a phone from them, you better do some thorough research in order to figure out, which phones use which networks.
Currently, I use Straight Talk on the T-Mobile network in Southern California. Speed and coverage are fine. I switched the network from AT&T to T-Mobile, though. The 25 GB in the $45 plan are more than enough for me. There is also an unlimited data plan. The only reason, why I think of switching to Metro by T-Mobile is the lack of hot spot data.
My tip
Activation kits in the store are often overpriced. So, order the Straight Talk SIM Kit for the network, which works for you and you phone, from Walmart's website, instead. They even ship it free to the store.
Yes, Straight Talk's support is not the best, but you hardly ever use it.
Conclusion
Straight Talk can be really good, if you know what you're doing. Choose the right network/phone combination and you're golden. - If you just grab a phone off the shelf without doing the proper research, you're pretty much gambling.