Yes, you still need to buy a data card. You will still need an Internet service provider in order to access the Internet.
This is not a substitute for unlimited access via a home network. But it is a good choice to use when traveling to avoid having to use unsecure public networks. Just be sure to set up some security because it defaults to wide open (see my review)
Yes, you will have to purchase service cards to keep this functional. Here are the plans available:
1 GB for $15 to be used within 30-days
2 GB for $25 to be used within 30-days
4 GB for $40 to be used within 60-days
5 GB for $50 to be used within 60-days
7 GB for $75 to be used within 60-days
This is "unlimited" in that it is only "limited" on how much service you want to purchase. If you have "unlimited" funds, then, yes, this is "unlimited" and will theoretically provide you with unlimited internet access.
Can this take the place of your home internet provider? It is not practical for primary home internet as 1) it operates only at 3G data speeds so streaming video or large downloads will be relatively slow, 2) latency is relatively high therefore it will not be ideal for gaming and other activities that require low latency, and 3) doing a lot of streaming, downloading large files, or doing other activities that require large amounts of bandwidth, will get quite expensive as you will need to buy a lot of service cards quickly.
Again, this device is intended more for those on the go that need an internet connection to do light surfing, check e-mail, do small downloads, check social media, etc. so that they have their own internet connection with them and will not have to rely on Starbucks, motels, airports, or any other public wifi to get an internet connection.