One thing you have to keep in mind is that unlimited still means a cap of about 5GB and I can use that up in just a couple of days. But then again, it all depends on what you do with your internet. For someone like myself, I will never replace my fios. I do a lot of online gaming, watching Hulu, downloading 1+GB files on a regular basis and I have a NAS at home and all my gaming consoles, NAS, network printer are connected to my home network.
Then you'd have to consider always having your phone charged since turning it into a hot spot can deplete the battery extremely quick and make the phone very hot. A mobile hotspot is nice for quick online when wifi isn't readily available, but I don't believe its a good replacement for your home connection, its just not too practicle and we don't know if Sprint will place any cap on 4G and most of the US won't be covered in 4G for a good while.
However, if you just browse the web at home every once in a while, check your emails or some youtube now and then, its a viable option. I'm paying an extra $20 for BES at the moment and getting a 25% off on my entire bill, so even if Sprint does charge an extra $20 for mobile hotspot which I don't believe is a pricey option, I'll obviously be ditching the BES option when I go with the Evo and tack on the hotspot option making my bill the same as now, about $73 with tax. NO ONE can offer me all that except Sprint thats why even though I was planning on ditching Sprint for the iphone this summer, my mind was changed when they announced the Evo. ATT would charge me over $100 a month and won't even include any SMS/MMS or mobile hotspot.