Personally, I have a 50 Mbps connection through a Verizon Fios internet only plan. They have competitive pricing for internet service. Furthermore, the quality of service is excellent, and the customer support is much better than other major competitors. See if they are available in your area by checking this promo page. I was able to get their service at a great price using that link.
The question of "when does it make sense?" can easily be rephrased as "how much money can I save?" and one of the biggest factors will be the price difference between combined internet and TV service or just internet. These are also the options that vary the most widely based on where you live, and the availability of promotions or contracts that can keep prices down for a short time.