Phibsboro is indeed run down around the awful shopping centre and Cabra Park (the slum property owned by the dismissed Garda Kevin Galvin was there). Like the rest of the city there also needs to be shop front legislation introduced as there is a lot of fly-by-night shops there.
On the plus side, the demolition of the shopping centre and replacement by something more aesthetically pleasing is a political priority, if not the political priority, in the area (the deal to take over Dalymount Park behind it and make a much larger centre fell through when the bubble burst). Once the Centre is gone, the whole area will have momentum. Additionally, some of the finest pubs in Dublin are there, including Porterhouse North, McGowan's (if you're young enough to be into going out) and The Gravediggers/Kavanaghs on Prospect Square in the Phibsboro side of Glasnevin. The Brian Ború/Hedigans and Smyths are also very solid, professional establishments. There are two massive GAA clubs in the area, Naomh Fionnbarra in Cabra and Na Fianna on Mobhi Road, as well as of course Bohemians' headquarters in Dalymount Park. The Botanic Gardens are a ten minute walk from Phibsboro Shopping Centre, while the Phoenix Park is a 20 minute walk in the other direction.
The area around Shandon Drive/Park/Road is the nicest residential part of Phibsboro, as a Google Streetview will confirm. Don't forget the Royal Canal goes right through the area making it a lovely place for walks and future redevelopment. Also, nobody has mentioned that the Luas North will go not only from Dublin city centre but from Grangegorman down Connacht Street to the Broombridge station in Cabra (where it will connect with the Iarnród Éireann line), making property in Phibsboro just a Luas trip for any student too lazy to walk. The transport links from this line will add to the investment value should you wish to rent it.