A cheaper alternative if you have a mid-2012 rMBP is to get an mSATA SSD and an mSATA to rMBP SSD adapter. The adapter can be had for between $10 and $20 depending on the source, and a Samsung 850 1TB mSATA SSD can be had for under $350 right now (although I got mine for under $300 about 6 months ago -- so it was a no-brainer to go this route instead of the JetDrive).
As an added benefit, you are not stuck with a semi-useless SSD if your rMBP ever dies: the mSATA can be used on other (PC) laptops or even a desktop with the proper adapter.
This upgrade breathed new life into my rMBP (with stock 250GB SSD) and I wish I had done it earlier. As for the Apple 250GB SSD, I bought a rMBP SSD to SATA adapter and stuck it in my home server.
But, to answer the OP's question, I think the 2012 rMBP is still a beast even to today's standards. The CPUs and (to a lesser extent) GPUs in the current lineup are not that much faster. The other enhancements such as battery life, WiFi and Thunderbolt 2 are nice, but fairly minor for my use. The only thing I sort of wish it had was 4k video output, and if you have only stock 8GB RAM, it may become an issue going forward. Honestly, I can't picture myself upgrading my machine for at least another two to three years.