I've not used raspberries in a wheat beer, but I have made a raspberry ale before. It turned out really well. I used 6ish lbs of raspberries in a 5 gallon batch. The beer was a beautiful color. Wait until primary fermentation is complete and rack onto the fruit in a secondary. A bucket is better. Also, bag the berries in a paint strainer bag. Another trick is to freeze and thaw the berries. This breaks down the membranes of the fruit.
I've done a peach wheat with fresh peaches. It wasn't good. It tasted very mudded.
Every July I brew a cherry stout with about 10 lbs or tart montmorency cherries. This beer is killer.
I've also brewed a tart cherry wheat using a high quality cherry concentrate. This is not an extract, but a concentrate.
http://www.shorelinefruit.com/cart/6/cherry-concentrate/-cherry-bay-orchards-tart-montmorency-cherry-concentrate---32-oz . This also goes in the secondary. This made a really good beer. I've also seen nice concentrate at my local farmers market too for roughly the same price.
I've made a lemon wheat with just using zest. It was a refreshing summer beer.
The key to keeping the fruit flavor is by adding the fruit at secondary fermentation. It is a true secondary fermentation, because you will get a secondary fermentation lol. But the fruit flavor won't get blown away as it would in the primary. When adding actual fruit, save a headache and bag it. I've never had an issue with head retention.