Well, they are not perfectly suited for home theater, however, I have been very happy with my Cerwin Vega Stroker 18. I actually had it in my previous car for about 5 years with a 2000wrms amp. About 5 years ago, I decided I wanted to use it in my home theater because it was very good at hitting the low notes. I don't need it to be 130dB, I mostly just wanted very deep bass that blends well with my other speakers. I no longer have those other speakers on the left side of the picture.
For reference, that box is 4ft wide, 20" tall, and 18" deep.
So, I had called Cerwin Vega and told them I wanted to use it for a home theater and wanted to know what size box and port to use, which they provided to me. I have it in a 7cuft box, tuned to approximately 23Hz, and currently only 120W per voice coil (so it's way under powered), but it sounds quite good. Well, that's what other people have told me. Of course, I like it otherwise I wouldn't still be using it. Eventually, I hope to get a much larger amp for it.
Someday, I will custom build another sub box, like a Maelstrom 21" or something.
I am curious to see what other people say, but I have seen some others that seems to enjoy using their old car subs for their home theaters.