Upsides:
In general (these are generalizations of course, but there's always some truth to it) Dominican women take better care of themselves (hygiene wise), trying to look good, dress well, etc...Example you have sex with a Dominican woman and she will always be clean down there, smell good, take a shower before and after, etc. (so they tell me...ahem).
Dominican women are carinosas (loving). They will hang on to you, cuddle up to you (i.e. you're driving around with them sitting on the passenger seat), caress you in front of whoever, etc.
Doing things for you such as cooking your meals and serving it to you, getting you the beer, taking your shoes off when you get home tired, ironing your clothes and making sure YOU also look good for them, etc...does not bother them (it is expected I guess)
They are of course great dancers. Nothing like dancing with a Dominican girl that will embrace you and dance with you as if you both were ONE. She will "grind you" (quemar) while looking at you in the eyes...
Dominican women will go with you drinking and partying all you want (if you take them) and don't complain if you go out and don't take them. You can get home any time you feel like and she will be there waiting for you and give you some loving regardless of whether she's a little teed off...which BTW I've found that no matter how mad you're at each other Dominican women in general won't withhold sex from you, contrary to American women who will use it as a tool to get what they want from their husband.
Downsides:
You marry a Dominican, you're marrying her family too. This can be good sometimes if you happen to get into a good family, but in general is more of a pain than anything.
You're always late because she's trying to look so good it takes her forever to get ready. No sense of urgency about anything...
As Chip already mentioned you can't be too good with them, as they will think of this as weakness and not respect you.
Many Dominican women are so used to the guy doing everything for them, they don't have a voice of their own. For instance you go to a restaurant and you ask what does she want "I don't know, whatever you want..." or "Whatever you order"...No I want a woman who has an opinion...
In general not a whole lot of ambition to get ahead and not have to depend from a guy.