Wanko Number One has two interwoven story lines, one which works and one which doesn't. Whether or not you should read this manga depends entirely on what you want out of this manga.
One storyline is "man sees his dog as a human, but the rest of the world sees her as a dog." While not especially original, this plot works very well. It is funny (hilarious at certain points) without actually crossing the line into beastiality or "ewww...." territory. The dog is cute (in both forms), the man is likable, the antics are great, and this part of the
story wraps up nicely.
The other storyline involves the same man getting dumped by his long-time girlfriend, and suddenly finding himself the center of a seinen harem manga. This plot doesn't work at all. It turns out that his girlfriend was cheating on him with another guy because he wouldn't propose, and left him because the other guy proposed to her. Talk about not caring who she marries... and yet, it's somehow his fault. He then flirts with a bunch of other girls (two of which are serious contenders), only to have his girlfriend dump the other guy and want him back (without ever addressing the whole cheating thing). He gets his happily ever after end (I won't spoil with who), but it all feels unsatisfying.
So, if you want a cute story about a cute dog acting human (some of the time), this is a great manga to read. Just breeze past the romance parts. If, on the other hand, you're looking for a good romance manga... keep looking.