The Sussex Spaniel is a hunting dog that is just being rediscovered as a potential family pet. In the United States, the hunters prefer larger dogs and don't need a smaller one like the Sussex Spaniel for underbrush game. So, despite their excellence at small game hunting, the dog isn't as much in demand by hunters in the United States. However, it is a beautiful dog with a genuinely friendly disposition. It can make a good family pet, if you understand that this dog is a hunting dog and has some of these traits bred into it.
One of the most obvious traits that you may find cute or annoying (depending on where you live) is that the dog will bay and howl. This is what the breed used when hunting game in England and it sometimes displays the behavior when it is upset or excited. The dog is used to running in packs and being with people, so it doesn't really fare well alone. It will suffer separation anxiety, which can come out as howling and baying or barking when you are gone from the house. This can annoy the neighbors, even if you think it's very cute.
The dog has large jowls that make it drool. It is a bit of a slob when eating or drinking anything. Some people find this trait very cute, but strangers might find it annoying if they get slobbered on. You can feed the dog outside and keep the slobbering and drooling outside for the most part too.
When it is exercised enough, the dog displays a calm almost dignified temperament. If it is not exercised enough, the dog will begin to chew destructively, potentially soil the house, and bark excessively. This dog wants to be outside romping and roaming to its heart's content.
The dog has a good temperament to be with children and other pets. It will try to dominate other dogs, but it can be socialized to be less insistent in getting its way. It is a very stubborn breed and does require obedience training to get the point across that it is the dog and you are the owner. The dog tends to forget its instruction and needs a bit of patience from a handler. It is not stupid, and has a good intelligence, but it is very single-minded about getting its own way.
The dog's coat does shed excessively, another good reason to keep it outside the home. The fur is perfectly suited for temperate climates and will protect it in wet weather too. Just have a small shelter for the dog to be in outdoors and will be very happy sitting outside guarding the backyard, as long as you are out there periodically to play with it.