val にも = が入っていたり、key/val に + があった場合に対応してみました。 val が不正で stringByReplacingPercentEscapesUsingEncoding が例外を出す場合もあるけど未対応。

NSURL+dictionaryFromQueryString.h

#import <Foundation/Foundation.h>

@interface NSURL (dictionaryFromQueryString)
-(NSDictionary *) dictionaryFromQueryString;
@end

NSURL+dictionaryFromQueryString.m

#import "NSURL+dictionaryFromQueryString.h"

@implementation NSURL (dictionaryFromQueryString)
-(NSDictionary *) dictionaryFromQueryString{

    NSString *query = [self query];
    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:0];
    NSArray *pairs = [query componentsSeparatedByString:@"&"];

    for (NSString *pair in pairs) {
        NSRange range = [pair rangeOfString:@"="];
        NSString *key = range.length ? [pair substringToIndex:range.location] : pair;
        NSString *val = range.length ? [pair substringFromIndex:range.location+1] : @"";
        key = [key stringByReplacingOccurrencesOfString:@"+" withString:@" "];
        val = [val stringByReplacingOccurrencesOfString:@"+" withString:@" "];
        key = [key stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        val = [val stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        [dict setObject:val forKey:key];
    }
    return dict;
}
@end